Whole-body x-ray bone densitometry using a narrow-angle fan beam, including variable fan beam displacement between scan passes

ABSTRACT

A whole body x-ray bone densitometry system includes a table extending parallel to a Y-axis of an XYZ coordinate system for supporting a patient at a patient position; an x-ray source for emitting a narrow angle fan beam of x-rays to irradiate at any one time a scan line which is transverse to the Y-axis and is substantially shorter than the width of a body cross-section of a typical adult patient occupying the patient position. An x-ray detector is aligned with the source along a source-detector axis which is transverse to the Y-axis, for receiving x-rays from the source within the angle of the fan beam after passage thereof through the patient position. The detector has a number of detecting elements arranged along a direction transverse to the Y-axis and to the source-detector axis. A source-detector support on which the source and detector are mounted at opposite sides of the patient position is coupled with a scanning mechanism moving at least one of the patient table and the source-detector support relative to the other to scan the patient position with the narrow angle fan beam in successive scans parallel to the Y-axis in which the source-detector axis is at different angles relative to the patient position as between different ones of successive scans, but in each of the successive scans, an origin of the fan beam in the source is at the same vertical distance from the patient table. The distance between the fan beam origin and the table may be selectively varied between scan passes, in order to vary scan pass magnification.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of parent application Ser.No. 08/345,069, filed on Nov. 25, 1994, pending which in turn is acontinuation-in-part of application Ser. No. 08/156,287, filed on Nov.22, 1993, now U.S. Pat. No. 5,452,834 both of which are herebyincorporated by reference herein as though fully set forth herein.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to x-ray systems and methods and more particularlyto x-ray based bone densitometry systems and methods and techniquesuseful at least in such systems and methods.

X-rays or gamma-rays can be used to measure the density and distributionof bone in the human body in order to help health professionals assessand evaluate projected bone mineral density, which in turn can be usedto monitor age-related bone loss that can be associated with diseasessuch as osteoporosis. Additionally or alternatively, similar procedurescan be used to measure non-bone related body content such as body fatand muscle. In bone densitometry, a patient typically is placed on atable such that the patient's spine extends along the length of thetable, along a direction that can be called the Y-axis in Cartesiancoordinates. For a supine patient, the left and right sides are in adirection typically called the X-axis. A source at one side of thepatient transmits radiation through the patient to a radiation detectorat the other side. The source and the detector typically aremechanically linked by a structure such as a C-arm to ensure theiralignment along a source-detector axis which is transverse (typicallyperpendicular) to the Y-axis. Both x-ray tubes and isotopes have beenused as a source of the radiation. In each case, the radiation from thesource is collimated to a specific beam shape prior to reaching thepatient to thereby restrict the field of x-ray or gamma radiation to thepredetermined region of the patient opposite which are located thedetectors. In the case of using x-rays, various beam shapes have beenused in practice including fan beam, pencil beam and cone or pyramidbeam shapes. When a fan beam is used, typically the beam conforms to abeam plane which is transverse (e.g., normal) to the Y-axis. Stateddifferently, the beam is wide in the plane and thin along the Y-axis.The shape of the beam and the shape of the detector system correspond.The detector in a fan beam system typically is an elongated array ofdetector elements arranged along a line or an arc. By means ofmechanically moving the C-arm and/or moving the table, a region ofinterest in a patient on the table can be scanned with the radiation.Typical regions of analysis in bone densitometry include the spine, hip,forearm, and wrist, scanned individually. They can be coveredindividually within a reasonable time by a fan beam that has arelatively narrow angle in a single pass or, alternatively, by a pencilbeam scanning a raster pattern. Another analysis region is termed"oblique hip" in which the hip is viewed at an angle relative to thehorizontal and vertical directions. Another analysis region is referredto as "whole body" in which the entire patient body is scanned andanalyzed for bone density and possibly also for "body composition" orthe percentages of fat and muscle in the body.

X-ray bone densitometry systems have been made by the owner of thisapplication under the tradenames QDR-2000+, QDR-2000, QDR-1500,QDR-1000plus, and QDR-1000. The following commonly owned U.S. patentspertain to such systems and are hereby incorporated by reference herein:4,811,373, 4,947,414, 4,953,189, 5,040,199, 5,044,002; 5,054,048,5,067,144, 5,070,519, 5,132,995 and 5,148,455; and 4,986,273 and5,165,410 (each assigned on its face to Medical & ScientificEnterprises, Inc. but now commonly owned). Other bone densitometrysystems are believed to have been made by the Lunar Corporation ofMadison, Wis. (see, e.g., the system which is believed to be offeredunder the tradename Expert and U.S. Pat. No. 5,228,068, 5,287,546 and5,305,368, none of which is admitted to be prior art against thisinvention). It is believed that other manufacturers also have offeredbone densitometry products.

The inventions disclosed in this application are directed toward bonedensitometry features which are believed to overcome variousshortcomings of such prior art systems.

In a particular exemplary and non-limiting embodiment, the inventionsare included in an x-ray bone densitometry system comprising a patienttable having a length extending along a Y-axis and a width extendingalong an X-axis, a C-arm supporting an x-ray source at one side and anx-ray detector at an opposite side of said table, the source anddetector being aligned along a source-detector axis which is transverseto the Y-axis. When selectively energized, the source emits a fan beamof x-rays which conforms to a beam plane which is transverse to theY-axis and contains the source-detector axis. At least one of said C-armand table is selectively movable relative to the other along the X-axis,along the Y-axis, and along a Z-axis which is transverse to both theX-axis and the Y-axis, to selectively scan selected regions of a patienton the table with said fan beam of x-rays. In addition, the C-arm isselectively rotatable around a rotational axis extending along theY-axis to selectively change the angle of the fan beam with respect to apatient on the table.

The present system has a scan motion controller which can move at leastone of the C-arm and the table relative to the other to scan at least afirst region and then a second region of a patient on the table with thefan beam along the Y-axis, wherein the first region and said secondregion are next to each other along the X-axis, each region has an edgeoverlapping an adjacent edge of the other region, and the distance alongthe Z-axis from an origin of the fan beam in the source to the tableremains the same for the scan of each of the first and second regions.The system can use a merger responsive to outputs of the detector forscans of the first and second regions to merge detector outputs forpositions of the fan beam which are spatially adjacent along the X-axisbut are obtained at different times, into resulting merged detectoroutputs corresponding to detector outputs obtainable from a single fanbeam having substantially twice the width of the fan beam emitted fromthe source.

The scan motion controller moves at least one of the C-arm and the tablerelative to the other to carry out first a posterior/anterior or ananterior/posterior scan and then a lateral scan of a patient on thetable without moving the patient between the two views. The scan motioncontroller rotates the C-arm around its rotation axis between carryingout the two scans, and selects the position of the source relative tothe table for the lateral scan based on information respecting thepatient's spine obtained in the course of the anterior/posterior orposterior/anterior scan.

One "whole body" scanning technique described in this and the parentapplications involves scanning the body in directions parallel to theY-axis in successive passes which are spaced from each other in adirection parallel to the X-axis. The effect is similar to that ofassembling a wider fan beam of x-rays from successive passes withnarrower fan beams. While the simplified analysis of the narrow fan beamassembly into a wider fan beam may suggest that the centerline of thenarrower fan is rotated about the focal spot hypothetical point oforigin F (hereafter referred to occasionally herein for brevity as the"focal spot") between successive passes, this is not the case in theactual practice of this whole body scanning technique. In fact, thefocal spot hypothetical point of origin moves in space betweensuccessive passes, along an arc centered at the center of rotation ofthe C-arm which carries the x-ray source and the x-ray detectors and thepatient table also moves in directions parallel to each of the X-axisand the Z-axis between successive passes. In addition, in the preferredembodiment, the angular spacing between the centerlines of the narrowerbeams is often not the same as the beam width, to cause an effectiveoverlap between the margins of successive beam positions.

A currently preferred "whole body" technique adds a differentprinciple--a selective displacement of the narrower fan between passesto make the x-ray imaging magnification factor significantly differentas between successive passes of the narrow fan of x-rays in a directionparallel to the Y-axis. Advantageously, this selective greatermagnification is at the spinal region or the left and right hip regionsof a supine patient centered on the patient table, whichevermagnification is desired for a particular medical diagnostic test.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention may be obtained from thefollowing description when taken in conjunction with the drawings, inwhich:

FIG. 1 is a diagrammatic representation of major subsystems of anembodiment of the invention.

FIG. 2 is a diagrammatic representation of mechanical subsystems of anembodiment of the invention.

FIG. 2A is a front view of a diagrammatic representation of one of themotorized drive systems for the mechanical subsystems, and FIG. 2B is atop view thereof.

FIG. 3A is an end-on view of a patient table and a C-arm of theembodiment of FIG. 2, in the position to perform a PA(posterior-anterior) spine measurement;

FIG. 3B is an end-on view for a hip measurement; and FIG. 3C is anend-on view for a lateral spine measurement.

FIGS. 4A and 4B are side elevational diagrammatic representations of therelative scanning motions made by the mechanical subsystems of anembodiment of the present invention and an equivalent motion thereof,respectively, when performing a whole-body scan.

FIGS. 5A, 5B and 5C are representations of x-ray fan beam coverage of apatient for whole body measurement, illustrating the use of a wide fanbeam made up or three passes or scans and involving notional rotation ofan x-ray tube around the focal spot from which it emits x-rays.

FIGS. 6A, 6B and 6C are end-on views of a preferred embodiment of theinvention for whole-body measurement showing the C-arm/patient tablepositioning for three measurement passes or scans.

FIG. 7A and 7B depict the relationship between the x-ray source andpatient table position for two measurement passes in accordance with anembodiment of the invention.

FIG. 8 depicts the relationship between the x-ray source and the patienttable for an oblique hip measurement in which the x-ray beam is angledrelative to the patient in a manner similar to that illustrated in FIGS.6A and 6B.

FIG. 8A is a representation of x-ray fan beam coverage of a patient forwhole body measurement, similar to that of FIG. 5B, illustrating the useof a wide fan beam made up or three passes or scans and involvingnotional rotation of an x-ray tube around the focal spot from which itemits x-rays.

FIG. 8B is a representation of x-ray fan beam coverage of a patient forwhole body measurement, illustrating the use of a wide fan beam made upor three passes or scans and involving notional rotation of an x-raytube around the focal spot from which it emits x-rays and selectivevariation of the distance from the focal spot to the patient table,which in this particular figure increases relative scan magnificationfor the central pass of the patient's spine.

FIG. 8C is a representation of x-ray fan beam coverage of a patient forwhole body measurement, illustrating the use of a wide fan beam made upor three passes or scans and involving notional rotation of an x-raytube around the focal spot from which it emits x-rays and selectivevariation of the distance from the focal spot to the patient table,which in this particular figure increases relative scan magnificationfor the lateral, outer passes of the patient's left and right hips.

FIG. 9 is a schematic axial view of a coaxial x-ray modulator of thepresent invention, shown in partial cross section.

FIG. 10 is a schematic radial view of the x-ray modulator of FIG. 9,shown in a single-drum configuration.

FIG. 11 is a schematic radial view of the x-ray modulator of FIG. 9,shown in a dual-drum configuration.

FIGS. 12A-12F show respectively the six rotational combinations of x-raymodulators which may be utilized in the present invention.

FIG. 13 is a controller block diagram for the x-ray modulator of FIG. 9.

FIG. 14 is a timing diagram for the dual-drum x-ray modulator of FIGS. 9and 11.

FIG. 15 is a schematic perspective view of an attenuator selection andpositioning mechanism of the present invention mounted in theexamination table unit of the present invention.

FIG. 16 is a detailed schematic perspective view of the attenuatorselection and positioning mechanism of FIG. 15.

FIG. 17 is a schematic perspective view of an optical crosshair linegenerating laser positioning aide of the present invention mounted inthe examination table unit of the present invention.

FIG. 18 is a detailed schematic perspective view of the opticalcrosshair line generating laser positioning aide of FIG. 17.

FIG. 19 is a perspective schematic view of a forearm positioning aide ofthe present invention.

FIG. 20 is a an elevational view of the forearm positioning aide of FIG.19, with a patient's arm positioned therein.

FIG. 21 is a plan view of the forearm positioning aide of FIG. 20.

FIG. 22 is a perspective view of a spinal positioning aide of thepresent invention.

FIG. 23 is an elevational view of the positioning aide of FIG. 22, witha patient positioned thereon.

FIG. 24 is a block diagram illustrating electrical and electronicsystems of an embodiment of the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

Scanning System Overview

Referring to FIG. 1, a scanning system 30 includes an examination tableunit 32 comprising a patient table 50 and a C-arm 56 serving as asource-detector support. Examination table unit 32 containselectromechanical components, control systems and other componentsinvolved in performing a patient scan and acquiring scan data. Scanningsystem 30 also includes a workstation 34 which controls the examinationtable unit 32 and C-arm 56 and processes scan data into forms moreuseful for diagnostic purposes, such as into patient images and reports.Workstation 34 includes a system power supply module 36, a host computer38 which has a floppy diskette drive recording device 40, an operatorconsole keyboard 42, and a display monitor 44, and can include anoptional printer 46.

Referring to FIGS. 2, 2A, 2B, 3A, 3B, 3C, 4A and 4B, a patient 48 canlie in the supine position during scanning on patient table 50. X-raysfrom an x-ray source 52 located beneath table 50 pass through patient 48and are received by a detector 54 having an array of detector elementslocated above patient 48. Each detector element responds to x-rays atrespective angular positions within a fan beam of x-rays. Both x-raysource 52 and detector 54 are supported on C-arm 56 which maintains aselected source-to-detector distance and alignment. In this example ofthe invention, x-ray source 52 has a stationary anode, and is adual-energy (DE) pulse system that is synchronized to the alternatingcurrent system power source.

A slit collimator 58 is between source 52 and patient 48. Collimator 58has one or more selectable slits machined or otherwise formed to allowthe passage of x-rays through a slit from source 52 to patient 48, andis made of an x-ray opaque material, such as lead or tungsten, ofsufficient thickness to substantially block the passage of x-raysthrough portions of the collimator other than the slits. For example,collimator 58 has a 1 mm wide collimator slit positioned an appropriatedistance from the focal spot in source 52 and suitably alignedtherewith. The x-ray radiation from x-ray source 52 passes through theslit in the collimator 58 and forms a fan shaped beam of x-rays 3a. Theangle subtended by beam 3a and the distance between its origin at thefocal spot of the x-ray tube and patient 48 are selected such that beam3a would not cover the entire cross-section of a typical adult patientat any one time but would cover only a selected portion of that width.Collimator 58 can have several slits which are differently dimensionedand/or shaped, and can be provided with a mechanism for aligning anyselected one of the several slits with source 52 and detector 54 tothereby select a desired shape for x-ray beam 3a. For example, each slitcan be long along the X-axis and narrow along the Y-axis, the severalslits can be in a row extending along the Y-axis, and the collimatorwith such slits can be moved along the Y-axis to align a selected one ofthe slits with the source and detector. In an alternative embodiment,collimator 58 can comprise a pair of x-ray opaque plates spaced fromeach other along the Y-axis to allow the passage of x-rays between themand thus to define the dimension of fan beam 3a along the Y-axis, andanother pair of x-ray opaque plates spaced from each other to allow thepassage of x-rays between them and thus to define the dimension of fanbeam 3a along the X-axis. The two pairs of collimator plates are coupledwith a control mechanism to selectively move them as required along theX-axis and the Y-axis to increase or decrease the dimension of fan beam3a along the X-axis and/or the Y-axis. Fan beam 3a can have a fan angleof 22 degrees, whereas a fan angle of, for example, 65 degrees may berequired to completely cover patient 48 for whole body analysis. Ofcourse, x-ray beam 3a not only has width (along the X-axis illustratedin the Figures) but also has a thickness along the Y-axis that isdefined by the width of the slit in collimator 58 (which can be, e.g., 1mm) and distance from the origin of beam 3a. A scan line is defined bythe portion of the patient imaged at any one time with fan beam 3a withdetector 54, i.e. the width and thickness of the x-ray beam over whichdata is collected at one point in time. While the term scan line isused, it should be clear than this "line" in fact is a rectangle thathas both a width in the x-direction and length in the y-direction. Acomplete pass or scan consists of a set of adjacent scan lines obtainedover a period of time such that the entire region of interest has beenmeasured. The scanning apparatus also has an x-ray beam modulator 60which is between collimator 58 and patient 48 and can modulate x-raybeam 3a in a periodic pattern for certain types of diagnostic scanning.There is also an adjustable x-ray beam attenuator 62 for changing theintensity and/or energy spectrum of x-ray beam 3a as desired fordifferent scans and/or other purposes.

System Scanning Motions

As seen in FIGS. 2 and 3A-3C, C-arm 56 rotates essentially within itsown volume along rotational path R about a rotational axis extendingalong the Y-axis. In addition, C-arm 56 moves along the Y-axis, alongthe length of a patient and thus along the patient's spine. The Y-axisand the Q-axis labeled in FIG. 2 extend in the same direction. C-arm 56includes a central portion 64 which can be formed of cast aluminum halfrings machined to a required rolling radius and combined with anintegrating structure to support x-ray source 52, slit collimator 58,x-ray beam modulator 60 and x-ray beam attenuator 62. A removable upperarm portion 66 houses x-ray detector 54, using a bracket interface.Thus, upper arm 66 may be removed for shipment in order to reduceshipping volume, and re-installed easily on site. A counter balancingsystem (not shown) is a part of C-arm 56, and is intended to minimizethe external forces required to rotate that portion of the device aswell as help balance C-arm 56, should a drive component fail.

Patient support table 50, as seen in FIGS. 2, 2A and 2B, is translatablealong all three axes--the longitudinal (Y axis), the transverse (Xaxis), and the vertical (Z axis). As seen in FIGS. 2A and 2B, table 50can be driven in the positive and in the negative directions along theY-axis by using a toothed drive belt 50a driven by a stepper motor 50bthrough a drive pulley 50c and an idler pulley 50c'. Belt 50a is securedto a table bracket 50d, which in turn is secured to table 50. A motorcontroller board 50e controls motor 50b. A DC servo motor can be used inplace of stepper motor 50b, and other drive implementations can besubstituted such as stepper-motor driven lead-screws. Each motion iscomputer controlled and monitored by an absolute encoder feedback systemreceiving feedback information from an absolute encoder 50f coupled withidler pulley 50c' to provide absolute information respecting anyrotation of that pulley and thereby respecting any motion of belt 50aand table 50 in each direction along the Y-axis.

C-arm 64 moves in conjunction with patient table 50. The motion of table50 makes it possible to achieve a more compact C-arm rotation volume.This can be seen by observing the geometric/volumetric motionrequirements seen in FIGS. 3A, 3B and 3C. The motions of table 50 in thetransverse and vertical directions (along the X-axis and along theZ-axis) help C-arm 64 clear table 50 when rotating between the threeillustrated positions of C-arm 64 used for different types of patientprocedures. In addition, the illustrated arrangement makes it possibleto keep patient table 50 as close as practical to x-ray source 52 duringposterior/anterior scanning while at the same time avoiding physicalinterference during rotation of C-arm 64.

As illustrated in FIGS. 4A and 4B, scanner system 30 makes it possibleto scan the entire length of patient 48, or any selected region of thepatient, as may be desirable in a "whole body" mode of operation, and atthe same time keep the Y-direction motion of C-arm 64 shorter than wouldbe needed if only C-arm 64 moved in the Y-direction. In this example,longitudinal scanning is accomplished by a combination of moving C-arm64 along the Q axis (which is parallel to the patient table Y axis) andadditionally moving patient table 50 in the longitudinal, or Y axis,direction. Each of C-arm 64 and table 50 moves a distance which is abouthalf the total length of patient 48. This reduces the total length ofthe scanning apparatus and thus reduces the clinical floor space neededfor the system. An illustration of this reduction in floor spacerequirement is seen when FIG. 4A is compared with FIG. 4B, which showsthe motion that would be required for a comparable scan along the lengthof a supine patient if only C-arm 64 moved in the Q (or Y) direction andtable 50 did not move in the Y-direction. This table 50/C-arm 64compound motion keeps the overall length of the scanning apparatus 30low when the system is not in the "whole body" scanning mode (and forthose machines not having the "whole body" feature), to thereby reduceboth installation size and shipping volume.

Another feature of scanning apparatus 30 is the method by which patienttable 50 is elevated and lowered in the Z (vertical) direction, as shownin FIG. 2. Z-direction motion is accomplished using two independentlymotorized telescoping pedestals 68, one at each end of patient table 50.Synchronization is important to maintain the telescoping pedestals in adesired operating mode, e.g., always extended an equal amount. This isaccomplished by employing an absolute linear encoder at each pedestallocation, similar to encoder 50f discussed above. A computer which is apart of the system interrogates each encoder in pedestals 68 duringmotion and modulates the power to the faster pedestal to maintain therequired synchronized motion by allowing the slower pedestal to catchup. This active synchronization is especially desirable in the case ofAC motor driven pedestals, since speed tends to vary with load. Evenwith other motor driven types such as steppers, such synchronization canbe of benefit, to ensure synchronous tracking even in the case of loststeps or other difficulties. The telescoping pedestals used in thisapparatus have a dual nut drive as an additional safety feature, in caseof drive failure. Each pedestal 63 can use a respective lead screw drivemechanism.

In addition, table 50 selectively moves left and right (as seen by asupine patient on table 50), along the X-axis. Table 50 is driven ineach direction along the X-axis under computer control by motors andlead screw or belt mechanisms in the upper portions 51 of pedestals 60,using motor control and absolute encoder feedback as described earlierfor the table motion along the Y-axis.

C-arm 56 rotates about a rotational axis which extend along the Y-axisand is at the geometric center of portion 64 of C-arm 56. It is drivenrotationally by a mechanism 57 (FIG. 2) and rides on rollers 72 (FIGS.3A-3C).

Scanner 30 includes two automatic positioning modes--PATIENT ON/OFF &HOME--which are activated by buttons on a table mounted control panel 70seen in FIGS. 1 and 2. The PATIENT ON/OFF function moves scanner table50 and C-arm 56 to positions that make patient loading particularlyconvenient, e.g., C-arm 56 moves along the X-axis all the way to theleft (as seen in FIG. 2) and patient table 50 all the way forward (inthe minus X direction seen in FIG. 2) and centered along the Y-axis. TheHOME function moves table 50 and C-arm 56 from their load positions (forthe PATIENT ON/OFF mode) to position suitable for starting a PA spinescan.

As carried on C-arm 56, x-ray source 52 and detector 54 have a 2-axismotion with respect to patient 48 to carry out scans. Motion in thelongitudinal Y (or Q) direction moves them along the patient axis asdefined by the spine. A second motion, along the R rotational path,rotates them around the patient, the center of rotation being at a pointC which is determined by the C-arm 56 and the method of rotationemployed. The point of rotation is not the focal spot in the X-ray tube;rather, the center of rotation is spaced from the focal spot by asignificant distance, and such spacing is important for the correctoperation of the system. In the preferred embodiment, x-ray detector 54and x-ray source 52, as carried by C-arm 56, rotate on a set of rollers72. Thus, the center of rotation "C" is determined by the outer radiusof C-arm 56.

As previously described, opposite x-ray source 52 is detector 54 whichin this embodiment comprises approximately 200 detector elementsarranged in a linear configuration extending along the X-axis in the XZplane. Detector 54 is about 16" long in the X direction and is about 42"from the origin of beam 3a (42" source-to-detector spacing) and subtendsa 22 degree fan angle. Alternately, the detector elements can bearranged along an arc centered at the focal spot in the X-ray tube. Thedetector elements that make up the array are silicon photo diodescoupled with a scintillation material, and they are fixed with respectto x-ray source 52. Other detector elements can be employed instead.

To perform a scan, a series of scan lines of data are acquired. To dothis, C-arm 56, carrying x-ray source 52 and detector 54, moves alongthe Y-axis along the length of patient 48. This motion moves detector 54and x-ray source 52 to form a succession of spatially overlapping scanlines adding up to a scanned rectangular area. The signals produced bythe detector elements in detector 54 in response to x-rays impingingthereon at successive scan lines are digitized by an analog to digital(A/D) converter and are stored, for example on disk. The host computer38 processes the signals from the A/D converter into densityrepresentations, and/or images, and/or reports of measured and/orcalculated parameters, using principles disclosed in the materialreferenced in the background section of this disclosure.

For body structures of interest such as the spine, hip, forearm andwrist, only a single pass of fan beam 3a along the Y-axis may be neededbecause typically the area of interest in the patient's body is coveredby fan beam 3a as shown in FIG. 3A for the Posteroanterior (PA) spineand in FIG. 3B for the hip. A similar scan can be performed on theforearm, as is done for the hip. Fan 3a has a sufficient angle to coverthe entire forearm and/or wrist of a typical patient in a single pass,thus completing the scan in substantially less time than would berequired for a pencil beam scanner in a raster fashion or by a narrowerfan beam which cannot cover the entire forearm or wrist in a singlepass. Indeed, in some circumstances a fan beam of only 14 degrees can besufficient for the geometry of this embodiment to fully illuminate anyof these body areas with x-rays. FIG. 3C shows the positioning for alateral scan of the spine in which the view is orthogonal to thestandard PA spine view. To attain this position, a series of movementsof C-arm 56 and table 50 are carried out to ensure that the table andC-arm clear each other. In this embodiment, table 50 is moved along theX-axis and the Z-axis appropriately, while c-arm 64 is rotated about anY-axis passing through point C until the desired lateral position isreached.

Whole body analysis can require that the entire body be illuminated withx-rays. Referring to FIG. 5A, a fan beam 3b of approximately 65 degreescan be suitable for completely illuminating the entire cross-section ofpatient 48. As illustrated in FIG. 5B, this fan beam can be simulated byutilizing multiple passes with a smaller, 22 degree fan beam 3a as longas the fan beam for all of the passes maintains a selected focal spot topatient body relationship. With a fan beam 3a of 22 degrees and thenominal dimensions of the system in this embodiment, three passes alongthe Y-axis can be made to cover the entire patient 48. Thus, data frompasses 1, 2 and 3 from the smaller fan beam 3a can be added togetherusing a computer to provide data that is substantially equivalent todata that would have been obtained if one large fan beam 3b had beenused. The conceptual illustration of FIG. 5B implies rotation of fanbeam 3a with the focal spot thereof as the center of rotation. With fanbeam 3a in a vertical orientation as in the middle position of fan 3a inFIG. 3B, fan beam 3a for pass 1 is rotated 21.5 degrees from thevertical while fan beam 3a for pass 3 is rotated -21.5 degrees from thevertical. The data from the 0.5 degrees of overlap is blended, e.g., byprogressively using more of the data from the next pass as one moves inangle toward the next pass, using for example principle known in secondgeneration CT technology.

FIG. 5C shows an enlargement of the area designated P in FIG. 3B, wherebeams 3a for passes 1 and 2 overlap spatially. Fan beam 3a is slightlywider than the required 21.5 degrees so that there is an overlap of 0.5degrees between the two passes. The overlapping areas imply that atleast two different elements of detector 54 have measured the x-raysattenuated through the same body area.

If rotation of beam 3a around its focal spot is desirable or practical,implementation of the multiple passes can be relatively easy because theonly required motion between passes is rotation. However, in thepreferred embodiment, the center of rotation C does not coincide withthe focal spot. In accordance with the invention, the focal spot is madethe effective center of rotation through motion of patient support table50. In the system in accordance with the invention, C-arm 56 and table50 can move with a total of five degrees of freedom. This feature isefficiently utilized in the whole body scanning mode.

Referring to FIGS. 6A, 6B and 6C, the three views depict the relativepositions of table 50 and C-arm 56 for three passes in the preferredembodiment of whole body scanning. Collimator 58 is not shown in theseviews. Each position maintains constant the spacing between the focalspot of beam 3a and table 50 as well as the location of a verticalintercept from the focal spot to table 50 relative to table 50.

FIG. 7A details the geometry of pass 1 in relation to pass 2. In pass 1,patient 48 lies supine on patient table 50 at position P1, and the focalspot of x-ray source 52 is at F1. in this position, only the left sideof patient 48 is illuminated with x-rays within fan beam 3a. If C-arm 56could now be rotated about the focal spot, the conditions of pass 2'would be achieved in which the central part of the patient 48 would beilluminated. However, the focal spot rotates about the center ofrotation of C-arm 56 located at C with a radius R. A rotation through anangle of -Θ about a pivot axis at point C attains the positioning ofpass 2 in which the focal spot is located at F2. To maintain the focalspot of beam 3a at the desired position relative to the patient, patienttable 50 moves to position P2 (without moving patient 48 relative totable 50). At position P2, the spatial relationship between F1 and P1are identical to the spatial relationship between F2 and P2, i.e., avertical drawn from the focal spot intersects patient table 50 at thesame point and extends over the same distance. To attain position P2requires two motions of table 50, one over a distance DX along theX-axis and another over a distance DZ along the Z-axis. These twomotions can be consecutive or concurrent (or can overlap in time only inpart). These distances DX and DZ correspond to the differences in X andZ coordinates for focal spot positions F1 and F2.

Referring to FIG. 7B, where the terms are graphically defined, thedistances DX and DZ are given by the relationships:

    DX=(X2-X1)=R cos φ(cosΘ-1)+sin φsin Θ!

    DZ=(Z2-Z1)=R sin φ(cosΘ-1)-cos φsin Θ!

Patient table 50 is translated along the X-axis over a distance DX andalong the Z-axis over a distance DZ, where φ is the angle that F1 makeswith the center of rotation C as the origin and Θ is the angle ofrotation between F1 and F2 which in the preferred embodiment is about-21.5 degrees, with the negative angle denoting a clockwise rotationaround C between passes 1 and 2. Similarly, for pass 3, the focal spotis translated by DX and DZ with Θ=-43 degrees.

As illustrated in FIG. 8, an additional analysis called the "obliquehip" can be performed in accordance with the invention by suitablyrotating C-arm 56 and translating patient table 50 along the X-axis andthe Z-axis. The actual position can be determined beforehand byperforming a "scout" scan which is usually a high speed, low dosage scanfor the PA hip. In FIG. 8, F1 is the location of the focal spot of beam3a, and line a-a' represents the field of radiation in patient 48, at adistance L from the focal spot of beam 3a. For convenience and clarity,patient table 50 is not shown in FIG. 6, but its position can be seen inFIG. 6A. A hip designated H1 is offset from the central ray of beam 3aby a distance D which can be quantitatively determined from the scoutscan. Upon rotation of C-arm 56 through an angle Θ (or 23 degrees in thepreferred embodiment) the focal spot is now at F2. Table 50 istranslated along the X-axis and the Z-axis while patient 48 remainsstationary on table 50 so that the patient's hip is at position H2 whichis now located in the central ray F2-H2 of the radiation field b-b' inpatient 48. In this geometry, the X and Z translations, DX and DZ, oftable 50 made to place the hip at H2 are given by the relationships:

    DX=R cosφ cos Θ!-sin φ R sinΘ-L!+D

    DZ= R sin φ+L! cosΘ-1!+RcosφsinΘ

where R is the distance of the focal spot F1 from the center of rotationC of the focal spot of beam 3a, and φ is the angle of the focal spot F1with respect to the center of rotation C. The distance L from the focalspot to the hip is estimated as the sum of the known distances from F1to the table plus the estimated distance from the table to the fielda-a'.

Selective, Variable Imaging Magnification Factor Between Scan Passes byFan Beam Displacement

A "whole body" scanning technique previously described herein involvesscanning the body in directions parallel to the Y-axis in successivepasses which are spaced from each other in a direction parallel to theX-axis. The effect is similar to that of assembling a wider fan beam ofx-rays from successive passes with narrower fan beams. A simplified wayof visualizing this technique is illustrated in FIG. 8A, where in onepass the centerline of a narrower fan is at position D1, for the nextpass the centerline is at D2 and for the third pass the beam centerlineis at position D3. At the level of the patient table, in the first passthe beam width is between points Ta and Tb, for the second pass the beamwidth is between points Tb and Tc, and for the third pass, the beamwidth is between points Tc and Td. In this simplified visualization, thecombined width of the beams used in the three passes is the widthbetween points Ta and Td at the level of the patient table. While thesimplified illustration of FIG. 8A may suggest that the centerline ofthe narrower fan is rotated about the focal spot hypothetical point oforigin F between successive passes, this is not the case in the actualpractice of this whole body scanning technique. In fact, the focal spotmoves in space between successive passes, along an arc centered at thecenter of rotation of the C-arm which carries the x-ray source and thex-ray detectors and the patient table also moves in directions parallelto each of the X-axis and the Z-axis between successive passes. Inaddition, in one embodiment, the angular spacing between the centerlinesof the narrower beams is not the same as the beam width, to cause aneffective overlap between the margins of successive beam positions.

Another "whole body" technique adds a different principle--adisplacement of the narrower fan between passes to make the x-rayimaging magnification factor significantly different as betweensuccessive passes of the narrow fan of x-rays in a direction parallel tothe Y-axis. This principle is illustrated in FIG. 8B in the frame ofreference of the patient table, i.e., without taking into account anymotion in space of the focal spot and of the table but only consideringrelative motion between the table and the focal spot F. In thisembodiment, for the first pass in a direction parallel to theY-direction, along the length of a supine patient on the table, thenarrow x-ray beam comes from focal spot F1 and is in position B1. Forthe second pass, the beam comes from focal spot F2 and is in positionB2. For the third pass, the beam comes from focal spot F3 and is inposition B3. In this embodiment, the vertical (Z-axis) distance betweenthe focal spot and the table differs as between focal spot positions F1and F2, and between focal spot positions F2 and F3. Consequently, themagnification factor is greater for focal spot position F2, assumingthat the focal spot-to-detector distance remains constant.Advantageously, this greater magnification is at the spinal region of asupine patient centered on the patient table, where often the bestmagnification is desired.

In the geometry illustrated in FIG. 8A, the angle between the passes Θis approximately twice the half-angle of the narrow beam, α, andtherefore the horizontal distance (Td-Ta) at the table level covered bythe three passes is:

    Td-Ta=2(T2-F)tan(3α),

where: α is the half-angle of the narrow fan,

T2 is the table center height along the Z-axis, and

F is the focal spot height along the Z-axis.

In the geometry illustrated for the currently preferred embodiment ofFIG. 8B, the corresponding width of the table (Td-Ta) is:

    Td-Ta=2(T2-F2)tan(3α)+2(FZ) tan(3α)-tan(α)!,

where: α is the half-angle of the narrow fan,

T2 is the table center height along the Z-axis,

F2 is the height of focal spot F2 along the Z-axis, and

FZ is the difference in height along the Z-axis between focal spots F2and F1, FZ=(F2-F1)sin(α), where F1 is the height of F1 along the Z-axis.

For the same patient table height relative to focal spot hypotheticalpoint of origin F1, a smaller half-beam angle is required to cover thesame table width in the currently preferred embodiment illustrated inFIG. 8B as compared with the geometry illustrated in FIG. 8A. In theFIG. 8B geometry, the magnification of objects at the table height ismore at the center and less at the sides of the table. The magnificationin the first pass, for beam B1, is (D1/T1)=(D2+FZ)/(T2+FZ), whereas themagnification for the second, central pass is simply (D2/T2). Theadvantages include higher image resolution and greater x-ray flux forthe central pass, which typically would include the patient's spinalregion.

The illustration of FIG. 8B is only for the relative positions of thefocal spot hypothetical point of origin and the patient table during thethree passes. In fact, in the preferred embodiment, these relativepositions are reached after coordinated motions in space of each of thefocal spot and the table. As illustrated in FIG. 7B previously describedwith respect to the other "whole body" scan motion embodiment which didnot achieve variable scan pass magnification, the motion of the focalspot is confined to a motion along an arc centered at C, which is therotational axis of the C-arm. In order to move the focal spot F fromposition F1, for the first pass, to position F2, for the second pass,the C-arm rotates about point C through an angle (-Θ). The distancesbetween F1 and F2 are given by:

    DX=(X2-X1)=R cos(Φ)cos(Θ)-1!+sin(Φ)sin(Θ)!

    DZ=(Z2-Z1)=R sin(Φ)cos(Θ)-1!+cos(Φ)sin(Θ)!,

where: DX is the distance along the X-axis between F1 and F2,

DZ is the distance along the Z-axis between F1 and F2,

R is the radius of the arc of the F1 to F2 motion,

Φ is the shown of R to F1 from the X-axis, and

Θ is the shown angle from F2 to F1.

For two successive passes to share a single ray without overlap, thecondition is (Θ=-2α).

For the geometry of FIG. 8B to be achieved in the preferred practicalsystem, the patient table moves in coordination with the rotation of theC-arm about axis C between the first and the second pass. This motion ofthe patient table is both along the Z-axis and along the X-axis, and isspecified by: ##EQU1## where TX is the motion of the patient table alongthe X-axis between the first pass with beam B1 and the second pass withbeam B2, and

TZ is the motion of the patient table along the Z-axis between the firstpass with beam B1 and the second pass with beam B2.

For the third pass, with beam B3, the angle or rotation Θ=-4α.

While the simplified illustrations of FIG. 8A and FIG. 8B the detectorsare shown along a single, horizontal line for all of the three passes,in practice the detector array remains perpendicular to the centerlineof the narrow beam. Thus, as discussed in said parent applications, thex-ray detector array would be angled relative to the horizontal for thepasses with beams B1 and B3. Similar software techniques as previouslydescribed herein can be used to provide appropriate geometriccorrections to account for this.

Yet another whole body scanning geometry is illustrated in FIG. 8C,which is similar to FIG. 8B in that the focal spot hypothetical point oforigin is at different vertical distances from the table as between thefirst and second passes, and between the second and third passes, butdiffers in that the magnification is greater for the first and thirdpasses than for the second pass. This can be desired where, e.g., in acase where the hips of a supine patient should be imaged at a greaterresolution and/or greater x-ray flux. In other respects, the geometry ofFIG. 8C can be achieved with the practical system disclosed in thisapplication in a manner corresponding to that described in theconnection with the whole body scanning of FIG. 8B.

X-Ray Beam Reference and Modulation System

A reference and modulation system 60 comprises a drum assembly 74 seenin FIGS. 9, 10 and 11, and a control system 75 seen in FIG. 13. Drumassembly 74 can use one nested cylinder 76 (FIG. 10) or two or morenested cylinders 78, 80 (FIGS. 9 and 11), or other shapes. System 60 isa three-dimensional rotating assembly, using support bearings for eachrotating drum, drive shafts, rotational position encoders, drive belts,drive motors with related pulleys, and attenuation material of differenttypes arrayed in a pattern within the inner periphery of one or moredrums. Control system 75 includes a controller which receives positionalsignals from an encoder and issues drive commands to the drive motorsystem.

Referring to FIGS. 9 and 11, drum assembly 74 has a pair of nested,preferably coaxial, hollow inner and outer cylinders 78, 80,respectively, on separate bearing sets 82, 84, respectively, which allowthe cylinders to rotate freely relative to each other. Shaft 86 forinner cylinder 78 does not extend into that cylinder, so that its centerremains hollow. Respective toothed pulleys 88, 90 are mounted on an endof each cylinder 78, 80, and they are connected via timing belts 92, 94to a single drive pulley 96 mounted on the modulator drive motor 98. Thepreferred ratios for pulleys 88, 90, 96 are such that outer cylinder 80would make one turn for three turns of inner cylinder 78, e.g., theratio of pulleys 96 and 80 is 1:1 while the ratio of pulleys 96 and 88is 1:3. Drive motor 98 can be a two-phase, pulse-width modulated (PM)stepper motor, such as one having 200 steps per revolution.

As seen in FIG. 13, encoder disks 100 and position encoders 102 (onlyone is shown for conciseness) for measuring the angular position of eachrespective cylinder 78, 80 are mounted at the opposite end of the drivesystem. Both encoders 102 and motor 98 are coupled to control system 75.

Within the inner periphery of each drum are the reference and filteringattenuation materials which are curved to match the drum inner radius sothat the path length of the x-rays through these materials would be thesame everywhere for any one attenuation material. The attenuationmaterials may be profiled to match the center of the fan beam radius, inorder to further equalize the path length of material traversed by thex-ray beam. As seen in FIG. 11, inner cylinder 78 is divided into four90 degree sections, with two brass strips 104 located 180 degrees acrossfrom each other. As inner cylinder 78 rotates, a sequence of: brass,air, brass, air, etc., at 50% duty cycle is generated. Both the brassand non-brass segments also contain the cylinder wall material, so theadditional attenuation value of the cylinder wall material may beaccounted for through scan data normalization. Outer cylinder 80 isdivided into six, 60 degree segments. At two opposing segment locationsare mounted bone simulating materials 106; another pair of opposingsegments have tissue simulating material 108, and the last two locationsare left empty and referred to as air segments 110. Rotation of outercylinder 80 therefore creates the following periodic sequence: bone,tissue, air, bone tissue, air, etc. As seen in FIGS. 12A-12F, when bothcylinders 78, 80 rotate in accordance with the previously definedcylinder rotational ratios, x-ray beam 3a passing through the center ofrotation would be modified by the following sequence of attenuationmaterials: bone+brass; bone+air; tissue+brass; tissue+air; air+brass;air+air; followed by a repeat of the same pattern for the second half ofthe outer cylinder.

Because the segments of like attenuation reference materials are located180 degrees opposite of each other, the x-ray beam traverses both piecesat the same time, eliminating the need to have the pieces criticallymatched. Another benefit of the coaxial drum 74 geometry is theminimization of the transition angle, defined as the angle during whicha non-zero width x-ray fan beam spans the edges of two materialsegments. The x-ray beam content is changing during the transition angleand is not desirable for patient scan measurements.

If desired, one, two or more cylinders may be nested, to vary the numberof attenuation material layers which intercept the beam path.

Modulator control system 75 is illustrated in FIG. 13 and comprises acircuit board having a microcomputer CPU 112 and interface circuitry.Control programs for operating microcomputer 112 are stored inelectronic memory, such as for example an EPROM memory device. Asuitable microcomputer is the model 80C320 manufactured by Motorola. Itshould be understood that other microcomputer architecture could beutilized to operate the controller. Control system 75 can be implementedin hardware only, without a CPU, or other known types of control systemscan be used having combinations of hardware and software processing, solong as they are capable of operating the modulator system in accordancewith the control parameters described in this specification. Inputs tothe system are commands from the host control computer 38; AC powerfrequency timing information from zero crossing detector 114; andpositional encoder 102 signals from drum assembly 74. Control system 75outputs are motor 98 step pulses to stepper driver electronics 113 andsystem status information to host control computer 38.

In operation, the rotational axis of modulator drum assembly 74 ispositioned along the long axis of the x-ray fan beam 3a throughmechanical alignment. As x-rays within fan beam 3a travel from source 52toward detector 54, they pass first through one wall of outer cylinder80, then through the material mounted on the inside of outer cylinder80, then through the wall of inner cylinder 78, then through thematerial mounted on the inside of inner cylinder 78, and so on, untilbeam 3a exits the other wall of outer cylinder 80, as shown in FIGS.12A-12F. When the two cylinders 78,80 are stationary, x-ray beam 3a ismodified by the composite stack of materials present in its path. Whencylinders 78, 80 are rotating, a sequence of different materialcombinations are inserted into the path of x-ray beam 3a in a periodic,repetitive fashion, as determined by the CPU-control 112 directing thedrive motor system. The sequence and/or timing of the materialcombinations which attenuate beam 3a can be modified by changingcontroller programming.

Through the use of the above-described ratios of modulator drive systempulleys 88, 90, 96 and through the use of suitable parameters forstepper motor 98, the system in accordance with the invention canachieve the timing relationships between pulses of x-ray source 52 pulseand positions of inner cylinder 78 and outer cylinder 80 illustrated inthe timing diagram of FIG. 14.

Attenuator Selection and Positioning Mechanism

FIGS. 15 and 16 illustrate the x-ray attenuator selection andpositioning mechanism 62 which is between x-ray source 52 and x-raydetector 54. The x-rays within fan beam 3a pass through attenuatormechanism 62, so that the effective beam intensity and/or energy(spectrum) are influenced by whatever attenuating medium is placedwithin the beam path.

Attenuator selector mechanism 62 includes a movable support plate 120which houses a number of materials 122 of varying thickness, physicalattenuation properties, or both, as desired or required for the imagingprocedures to be performed by system 30. As seen in FIG. 16, materials122 can be arrayed next to each other in the Y direction, with eachindividual material extending in the X direction. Alternatively, otherarray patterns can be selected, such as a radial, planar, or a threedimensional array that envelops the x-ray source 52. However, a flatplanar array of sequentially placed materials, similar to a laminatedbutcher block table, provides for cost effective manufacture within asmall, flat package. Low system profile of the selector mechanism,located as close as practical to the focal spot in x-ray source 52,reduces the physical size required for each block of attenuatingmaterial to cover the entire imaging beam 3a, thus reducing materialcost and weight. Support plate 120 is supported by and slides on maindrive plate 124, which in turn is coupled to C-arm 56. The relative fitof support plate 120 and drive plate 124 provides lateral alignment ofthe attenuation materials relative to x-ray beam 3a.

Support plate 120 in attenuator mechanism 62, and the attached array ofdifferent attenuating materials 122, are coupled to a drive mechanism126 for translation relative to radiation beam 3a. As seen in FIG. 16,drive mechanism 126 includes a motor bracket 128 attached to slidingsupport plate 120. A linear motor 130 is attached to motor bracket 128and a drive screw portion 138 of a linear motor is rotatively attachedto main drive plate 124, to cause the relative sliding motion betweenmain drive plate 124 and support plate 120. Other suitable drivemechanisms can include a rotary stepper motor with a cogged belt drive,worm gear mechanism, drive screw mechanism as used in machine tool beds,or any other type of known drive system which can provide the desiredrelative sliding motion between support plate 120 and main drive plate124. It is also possible to utilize a manual drive mechanism, such as ascrew jack cranked by the machine operator. A rotary encoder 134 isattached to motor bracket 128. This rotary encoder 134 has a pinion gear136 interacting with a gear rack 138 mounted on main drive plate 124. Inthis manner, the rotary position output of encoder 134 can be correlatedto the position of a specific attenuation material 122 relative toradiation beam 3a.

A controller 140 (see FIG. 24) reads the output signal of attenuatormechanism encoder 134 and also provides drive signals for actuation oflinear motor 130, in a manner similar to that discussed in connectionwith x-ray beam modulation system 60. Thus, when the scanner operatorselects a desired attenuation material 122 by way of the scanner controlsystem, the scanner automatically aligns the desired material 122relative to the radiation beam path 3a. Alternatively, other motorcontrol and drive systems well known in the art may be utilized inconnection with the attenuator drive mechanism.

Optical Crosshair Line Generating Laser Positioning Aide

The x-ray system described herein has the capability of measuringvarious anatomical regions, and includes an optical crosshair devicewhich helps the operator position the patient on table 50. The operatoruses the crosshair device to ensure that the x-ray beam will be directedto the desired anatomical region, that different scans will registercorrectly with anatomical features or with each other, and that scans ofthe same region but at different times will register well. Accuratepositioning helps avoid the need to interrupt a procedure when itbecomes apparent that the measurements being obtained are not for thedesired anatomical region, or to repeat procedures for similar reasons.It also helps achieve reproducible positioning of the anatomy, allowingbaseline scans to be used reliably for subsequent scan evaluations.

As illustrated in FIGS. 17 and 18, a single line projection laser 152 isthe source of the laser beam. When C-arm 56 is in the positionillustrated in FIG. 17 (for a posterior-anterior scan), the laser beamis directed downward, creating a visual crosshair beam 160 consisting oftwo fan beams of laser light approximately ninety degrees to each other.Crosshair beam 160 can illuminate a patient, or the top of table 50, ora calibration device. Although the laser is low voltage the line qualityof crosshair 160 is bright and crisp, even in a well lit room. The lowprofile, tri-pod adjustment, and internal shutter permit the laser to beinstalled in tight fitted areas but still allow for ease in adjustmentor replacement.

The optical crosshair device is constructed of a one piece base 144, twooptical mirrors 146, 148, a beam splitter 150, one optical linegenerating laser 152, and a internal mechanical shutter 154 with anexternal slide 156, allowing the operator or the patient to block thelaser beam. The external tri-pod adjustment 158 permits initial laseralignment to the array and the source. The Y axis fan beam of laserlight of crosshair 160 aides in aligning the patients spine along the Yaxis of the x-ray apparatus. The X axis fan beam of laser light ofcrosshair 160 helps align the hips perpendicular to the spine and thusto the Y axis of the x-ray apparatus.

Forearm Positioning Aide

For a wrist or forearm scans, it is desirable that the patient's wristand/or forearm be suitably oriented relative to the scanning x-ray beam3a, e.g., with the forearm extending in the Y direction, and with theradius and ulna bones side-by-side in the X-direction. It is alsodesirable that the forearm and/or the wrist remain in one positionduring the scan, and that the positions be accurately reproducible forsubsequent scans so that baseline comparisons can be made.

Referring to FIGS. 19-21, a forearm positioner 164 can be used with thescanning system described herein (as well as with pencil x-ray beamscanners). Forearm positioner 164 can be constructed of polycarbonatematerial, such as LEXAN, manufactured by General Electric Company, andan x-ray translucent material. It has a base portion 168 with an inboardside which faces the patient and is covered with a polyester foam layer169 to make it more comfortable for the patient. At the outboard end ofbase portion 168, a ridge 170 can be constructed of a wedge-shaped pieceof polyester foam which extends upwardly to aid in positioning thepatient's forearm. Forearm positioner 164 has a cut-out portion 172which is generally parallel to and proximal to and just inboard of theridge portion 170. Forearm positioner 164 clamps over a side edge oftable 50 with clamping lip portion 174. During a forearm or wrist scandata acquisition, positioner 164 is at a fixed, centrally locatedposition on table 50. The patient sits beside table 50, with the armover table 50 and positioner 164, and presses his or her forearm 166down on base portion 168 and outward against ridge portion 170, as shownin more detail in FIG. 20, with the anatomical area to be scanned beingover the cut-out portion 172 so that the positioner 164 would not affectthe x-ray intensity measurements.

Spinal Positioning Aide

Referring to FIGS. 22 and 23, a spinal positioning aide 180 can be usedwhen performing spinal scans, such as the PA and lateral scanspreviously described. Spinal positioner 180 is preferably constructed ofx-ray translucent polyester foam and is covered with a removablematerial. Spinal positioner 180 helps support and position the patient'shead, arms and upper shoulders in comfort and in positions which helpsthe spine portion which will be measured relax and extend relativelystraight in the Y direction on table 50. Often, two scans are performed,one in a posterior-anterior projection and one as a lateral scanapproximately ninety degrees from the first projection. The first scanobtains information which helps in carrying out the second scan. It isdesirable for the accuracy of the measurement that the patient remain inthe same position for both scans and that the patient's spine and hipsbe suitably oriented relative to scanning x-ray beam 3a.

Spinal positioner 180 has a base portion 182, with an radial indentation184 therein which extends in the Y direction and helps support thepatient's head, neck and hands. A ramped portion 186 helps support theupper shoulders and the neck. Wings 188 extend upwardly and divergelaterally away from base portion 182 to help support patient's arms suchthat the elbows are elevated.

For good ergonomics, spinal positioner 180 is shaped to fit the naturalshape of a person who may have to remain in the position illustrated inFIG. 23 for some period of time. The angle of the ramped portion 186,which supports the patient's upper back and neck, fits the desiredcurvature of the spine. A drop off at the top of the ramp 186, intoindentation 184, helps support the neck and head. The angular cuts inthe wing portions 188 allow several different arm positions and preventthe patient's arms from rotating too far above his or her head tothereby reduce patient discomfort.

With the arms above the patient's head, as illustrated in FIG. 23, thepatient's rib cage tends to rise and the scapulas tend to rotate out oftheir normal positions. This helps achieve a clearer projection of theupper thoracic spine region. Positioner 180 is preferably covered with amaterial that is fluid proof, bacteriostatic, and removable (such as viahoop-and-loop fastening material), so it can be easily changed for a newpatient.

Scanner Electrical and Electronic Control Systems

FIG. 24 illustrates, in block diagram form, scanner electrical andelectronic control systems of an embodiment in accordance with theinvention. Examination table unit 32 includes the structure illustratedin FIGS. 1 and 2, as well as a suitable power supply module 36 for x-raysource 52 and motors for driving patient support table 50 and c-arm 64,and to operate attenuator 62 and modulator 60. Each of the motors has alocal controller with motor driver electronics and position encoder,similar to those used in the x-ray modulator system shown in FIG. 13.For the sake of conciseness, each of those local elements is notrepeated in this figure. In FIG. 24, the drive system XX which causes Xdirection translation of patient table 50 is shown as including a motor200, a motor position encoder 202 and local X motion controller/motordriver electronics 204. For the sake of brevity, similar structure forthe Y direction translation of the patient table is shown as block YY,and Z direction patient table translation as block ZZ. Block RR of C-arm56 (including c-arm 64) depicts the c-arm rotation drive system, withlocal controller, and block QQ denotes the c-arm translation in the Qdirection (which is the same as the patient table 50 Y direction). Thelocal controllers for drive systems XX, YY, ZZ, QQ and RR communicateover motor bus 206.

As further shown in FIG. 24, the C-arm 56 has a C-arm local controller208, which communicates with x-ray source controller 210, the x-raymodulator controller (which includes CPU 112), x-ray attenuatorcontroller 140 and control panels (212, 70) which are located in theC-arm and patient table, respectively. C-arm controller 208 communicatesvia C-arm controller bus 214.

Detector array 54 supplies x-ray measurements to data acquisition system(DAS) 216, where the measurements are collected and can be preliminarilyprocessed. DAS outputs 216 its collected and processed x-raymeasurements from the individual elements of detector array 54 via DASbus 218.

Digital Signal Processor (DSP) 220 is coupled to each of the motor bus206, C-arm controller bus 214, and DAS bus 218, and functions as acommunications exchange for the remote controllers with host computersystem 38. While use of a digital signal processor 220 is shown in thisembodiment, it is contemplated that any known system which can networkcommunications between the various local processors and the hostcomputer 38 can be used in connection with this invention. DSP 220includes an interface for communication with the host computer inconventional fashion, such as by an ISA bus or through an industrystandard interface on the card (e.g., SCSI, IEE488, etc.) to acommunications line 222.

Use of distributed processing and communications networking between aplurality of local processor controllers via the DSP 220 interface,reduces wiring complexity between various controlled devices and thehost computer system 38. DSP 220 is responsible for real-timeprocessing, such as motion control over table 50 and C-arm 56. Hostcomputer 38 also has the advantage of having a more integrated andconsistent datastream content in the DSP 220 data buffers than would becommunicated by all of the separate local controllers. For example, bothscan data from the DAS 220 and its corresponding position data obtainedfrom the scanning system patient table 50 and C-arm 56 position encoders(e.g., 202) can be contained in the same data buffers.

Host computer 38 provides central command and control 230 of the entirescanner system. In the embodiment shown herein, host computer 38 is anIBM AT-compatible architecture computer, having therein an 80486/25 MHzor higher clock rate microcomputer, manufactured by Intel or equivalentvendor product.

In order to perform scan data processing, the ultimate goal of thescanning system, scan data from the DAS 216 is forwarded to the hostcomputer 38, which is programmed to perform A/D conversion at 224 andpreliminary data preprocessing at 226 similarly to said QDR-2000 andQDR-2000+ systems. The output of the preliminary data preprocessingfunctions 226 is supplied to another image processing program 228, whichperforms various calculations and forms an image in a manner similar tothat used in said earlier systems and, additionally, blends the datafrom successive scans (using among other things, the patient table andC-arm positional encoder data) in a manner similar to that used insecond generation CT technology to form whole-body images. While the A/Dconversion 224, preprocessing 226 and image processing 228 functions canbe performed by the host computer 38, executing program modules, thosefunctions can be performed in separate, dedicated data processingapparatus.

Data and images from processor program 228 are supplied to a console 42,display 44 and a recorder (e.g., floppy disk drive 40 and/or a printer)via user interface 232 for purposes and in a manner similar to those insaid earlier systems. Two-way arrows connect the elements of FIG. 24 toillustrate the fact that two-way communications can take placetherebetween. Conventional elements have been omitted from the Figuresand from this description for the sake of conciseness.

All of the above described mechanisms are controlled and coordinatedunder computer control (local controller or the host computer 38). Eachmotion of the apparatus is monitored by an absolute encoder feedbacksystem. All motions, except for the telescoping pedestals 68 used toraise and lower the patient table 50, employ absolute rotary encodersthat do not require zero switches as would be required with incrementalencoders which can only count motion from a known starting position. Theuse of slow speed, continuous loop belt drives for all motions exceptthe telescoping pedestals 68, makes this technique practical. Otherapparatus which employ high speed lead screw drives do not lendthemselves to this simpler, absolute encoder technique because of thelarge number of revolutions required by the drives for positioning.Absolute encoders are restricted to a finite number of revolutions tostay within their operating range. The encoders are located and directlyconnected to the idler take\up pulley shaft which only rotateapproximately 8 turns out of 10 allowed by the encoder during fulltravel for each of the various mechanism.

The use of position encoders, such as absolute encoders, is importantfor monitoring and ultimately controlling the motion control systems ofscanner apparatus 30. The close proximity of the structures and thepotential for collisions with one another does not lend itself as wellto mechanisms moving to locate zero switches to determine the locationof each element of the system during power up. An important feature ofabsolute encoding is that location knowledge is never lost during powerdown/power up.

Scanner System Operation

As was previously described, x-ray source 52 is a dual-energy (DE) pulsesystem that is synchronized to the alternating current (AC) powersource. Rotating drum cylinders 78, 80 on modulator 62 also aresynchronized to the AC power line by way of modulator controller 112,which implements a closed loop control sequence. Review of the timingdiagram of FIG. 14 will assist in understanding the scanning x-ray pulsesequence and modulation.

Referring to the timing diagram of FIG. 14, ACLINE represents a squarewave derived from the AC line frequency (60 Hz in the United States).The term SEQUENCE describes the three energy states of the x-ray source;that is "B" for black, or no energy output pulse, "H" for the highenergy emission pulse, and "L" for the low energy emission pulse. Theterm SEGMENT means the attenuation materials described as lining themodulator outer cylinder 80. Similarly, BRASS and AIR mean thealternating strips of brass attenuation material, and no attenuationmaterial (i.e., "air"), along the modulator drum inner cylinder 78.SEGMENT PICKUP and INDEX signify respectively the inner 78 and outer 80cylinder position encoders (102) output signals that are used by thecontroller in feedback mode to synchronize drum rotation to the AC powerline frequency, and thus the x-ray source 52 energy pulsing sequence.MOTOR STEP means each step pulse command issued by the controller CPU112 to the stepper driver electronics 113, so that the stepper motor 98advances an additional rotational increment.

Modulator controller CPU 112 accepts commands from the host computer 38to operate the modulator 60 in one of two modes: continuous orpositioning. In the continuous mode, the stepper motor 98 for themodulator 60 is accelerated from a stopped position to a constantrunning speed, which is a function of the AC power frequency and thex-ray pulse mode. In the positioning mode, the stepper motor 98 iscommanded to rotate until the modulator drum inner and outer cylindersare in a desired position, as determined by the modulator rotationalposition encoders 102. Once the desired stationary drum position isattained, to have the needed attenuation media aligned within the x-raybeam path, the motor 98 remains energized sufficiently to preventinadvertent drum movement, i.e., analogous to using the motor as anelectromechanical brake.

When the operator starts the continuous mode of system operation, themodulator controller CPU 112 determines the AC power frequency 114 andcalculates the step rate required to operate the motor 98 at a fixednumber of x-ray pulses per cylinder segment. The step rate is generatedfrom an internal timer that counts ticks of the CPU 112 clock frequency.A parabolic acceleration spiral is calculated that will "soft" start themotor 98 at a slow speed, (within the motor's starting currentspecifications, so as not to overload it), and accelerate it to thecalculated running speed. The "soft" motor start acceleration profile istailored to reduce the required starting torque; therefore motor sizeand drive system wear and tear are also minimized.

Modulator controller CPU 112 also calculates a nominal phase angle,between the AC line frequency and the modulator drum starting positionindicated by the cylinder encoders 102. The modulator control system 75then slowly steps the cylinders 78, 80 to a zero phase angle, determinedby processing the encoder 102 output signals; it also sets motor powerlevel up to a value required for smooth acceleration. Next, the CPU 112waits for the next AC power line zero cross signal 114, then starts inACCELERATE mode, bring the stepper motor 98 and cylinders 78, 80 up torunning speed. When the motor and cylinders are at the final runningspeed, the CPU switches to a LOCK mode. Each time that the AC power hasa zero cross, the timer that generates the step pulse frequency is resetand restarted. This reset causes the timer to discard any smallvariation between the crystal oscillator of the CPU and the actual ACpower frequency. The stepper motor 98 can respond to small, but quickchanges in the step rate, enabling synchronized cylinder and powerfrequency.

When the modulator motor 98 is in synch with the AC power frequency, thephase angle between the cylinder attenuation material passage throughthe x-ray beam path and the x-ray generator pulse is adjusted. As themodulator cylinders rotate, the modulator controller 75 reads back theactual cylinder positions from the encoder 102 position signals andcompares the delta time between the start of a new attenuation materialsegment and the start of an AC power line duty cycle. For a given deltatime, the CPU 112 can measure the phase angle between the start ofcylinder attenuation material segments and the x-ray pulses.

In order to adjust the phase angle between start of cylinder attenuationsegments and the x-ray pulses to a desired value, the modulator controlsystem 75 makes a small calculated change to the step rate timer foradvancement or retardation of the phase angle. Once the phase angle isadjusted to be within programmed tolerances, the controller 75 sends astatus message to the host computer 38, indicating a LOCKED condition.The CPU 112 continuously monitors the AC power frequency and thecylinder encoder 102 position signals to make timing adjustments.

The closed loop control of the pulse rate for stepper motor 98 inmodulator system 60, using the AC power frequency as the referencefrequency, offers advantages which include:

relatively lower cost of stepper motors compared to larger synchronousmotors utilized in prior art modulator systems;

elimination of the need for high accuracy, expensive tachometers orencoders;

no need for linear servo motor systems;

the stepper motor serves a dual function as a stepper positional devicewhen the scanning apparatus is operated in positioning mode, i.e., onlyone set of attenuation material layers is needed for a particular typeof scan; and

stepper electronic control systems are relatively inexpensive toimplement.

Detector Calibration

The individual elements of the detector 52 are corrected fornonuniformities with angle in the fan beam and for beam hardening fordifferent intensities. Each element of the detector 52 is alsocalibrated for offset and gain by taking dark level scan detectorelement readings which are interspersed with patient scan readings in asampling pattern of On and Off x-ray pulses.

A. Continuous Dark Level Sampling

The system alternately turns X-rays on and off and this makes itpossible to intersperse dark level measurements with x-ray signalmeasurements. The x-rays may be cycled on and off according to differentschemes such as Off, On, Off, On . . . or Off, Off, On, On, Off, Off,On, On , etc. During each On cycle, the x-ray signal is measured; duringeach Off cycle, the dark level offset is measured. The dark level offsetcan be subtracted from the time-adjacent x-ray signal measurement(s), ormultiple dark level offsets can be averaged, and the average subtractedfrom multiple X-ray signal measurements.

An exemplary embodiment is shown in the timing diagram of FIG. 14,wherein an Off, Off, On, On, . . . sequence is utilized. Moreparticularly, the respective outputs of the x-ray detector elements indetector 54 for the two Off pulses are measured (signified by the letter"D" in the timing line SEQUENCE). Thereafter, the same measurements aretaken for two pulses at a first energy level (H for "high"). Thereafter,the measurements are taken again for two Off pulses, then for two pulsesat a second energy level (L for "Low"). Thereafter, two more Off pulsesare measured. The sequence is repeated many times during the course ofthe patient scan. Twelve offset measurements are averaged to determinethe dark level offset that is subtracted from each of twelvetime-adjacent X-ray signal measurements. As a result of this feature ofthe invention, if the dark level offset varies over time, this will beaccounted for correctly since the dark level offsets are measured atnearly the same time as the x-ray signals from which the offsets aresubtracted. Second, a dark level offset is measured over the same timeduration as the x-ray signal. Thus, the dark level offsets are measuredat photon statistics corresponding to those for the x-ray signals.

B. Multiple Thickness Beam and Detector Flattening

In the preferred embodiment, variations in x-ray beam characteristicsare accounted for through the use of a multiple thickness flatteningsystem. The system utilizes the attenuator selector mechanism 62previously described herein to take calibration readings automaticallyfor different attenuation media under control of the host computer 38.

The flattening procedure involves collecting data representative of oneor more of the modalities of which the system is capable. Referenceattenuation at multiple thickness levels, and thus attenuation levels,is achieved either by means of the internal attenuator mechanism 62 orby the use of a phantom block that can be positioned between the x-raysource and detector. Data are processed by the x-ray system's computerto produce specific factors that are stored permanently for later use.Values that are stored include reference values corresponding to eachattenuation level and correction factors for every detector channel ateach attenuation level. Such correction factors may be calculatedrelative to one detector selected as the "reference", the average ofmore than one "reference" detector, alternative reference data, or otherspecified attenuation levels.

Scan acquisition software utilizes stored flattening data to makecorrections to the original input data in real time as they areacquired. Alternatively, software can provides a way to store theoriginal data and apply the flattening corrections at a later time. Theexact correction for each datum point can be interpolated orextrapolated from the multiple level correction factors, based on theattenuation level relative to the reference attenuation levels. Variousinterpolation and extrapolation methods and algorithms can be applied tomodel the response of the system. Piece-wise linear interpolation andextrapolation offer the preferable characteristics of sufficientaccuracy with minimal computational intensity.

C. Flattening Update

Changes in the x-ray distribution and detector gain characteristics ofthe system can be monitored and adjusted by means of subsequentflattening scans. Comparisons with earlier initial flattening data canprovide diagnostic information and a means to make adjustments. Thesystem can be configured to perform and analyze flattening scans on aregular, periodic basis. Moreover, a flattening scan acquired with noadded attenuation, using all of the channels of detector 54, can becompared to one taken at the time of an earlier flattening procedure.Differences calculated on a respective detector element by detectorelement basis are applied to adjust gains in other scan modes. Thereby,drifts in gain levels can be canceled. Diagnostic information obtainedthrough a flattening update allows for software-controlled determinationof possible systematic drifts in x-ray output, changes in filtration,variations in machine geometry, or detector failure.

Limits may be set in the calibration software configuration for averagedrift and detector non-uniformity. If these limits are exceeded, thenthe operator is warned and further normal scanning may be disabled. Inthe case where a broken detector channel can be recognized, that channelmay optionally be eliminated and replaced by interpolated values fromits neighbors.

D. Exemplary Detector Calibration Calculations

In the preferred implementation, offsets of respective detector 54element offsets can be accounted for in a linear data representation,while beam and detector flattening corrections can be applied in alogarithmic data representation. Detector offsets are subtracted fromthe x-ray measurement data while in linear space. After offsets aresubtracted, the data are transformed to logarithmic space for subsequentdata processing and analysis. After taking the log, the attenuation at agiven x-ray energy becomes linearly proportional to the x-ray thicknessof a given isotropic material. In the logarithmic format, gainsdifferences in the detector system can also be compensated throughaddition and subtraction. The following equation describes the dataoperations that are used to produce a flat image with a fan beam,multiple detector x-ray system as in the invention disclosed herein:##EQU2## where: detector! is the detector channel index,

attenuator! is the attenuator block index,

FLAT is the resulting flattened aid calibrated data,

RAW is the original data (logarithmic form with offsets removed), OFFSETis the detector dark current offset,

REF is the reference attenuation array,

FACT is the array of flattening factors,

DIFF is the array of calibration differences,

SLNUM is the numerator of the slope, and

SLDEN is the denominator of the slope.

The "detector" index is applied to each detector channel in the system.The "attenuator" index is chosen such that the reference attenuation forthat attenuator is the greatest that is less than the attenuation valueof the original datum. Thus, there is linear interpolation when theinput is between reference values and extrapolation when the attenuationvalue of the input exceeds the thickest attenuator.

Simultaneous Single- and Dual-Energy Imaging

Single-energy and dual-energy scans may be reconstructed from the samescan data sets and displayed simultaneously on the display monitor 44 inaccordance with the invention. The scanner 30 when making dual-energyscans can store the scan sets taken at the high energy levels separatelyfrom the data taken at the low energy levels.

Spatially synchronized single-energy and dual-energy images are acquiredby passing the C-arm 56 over the anatomical area of interest. The scansets obtained at both energy levels are processed to extractdensitometric information. The previously-described positional encodersin the XX, YY, ZZ, QQ and RR drive system controllers (FIG. 24) allowprecise spatial registry of scan sets taken at both energy levels. Thesingle-energy scan data can offer better spatial resolution and signalto noise characteristics than dual-energy scans. Thus, densitometricmeasurements as well as geometric measurements can be displayedsimultaneously on the display screen 44 for evaluation by the medicalpractitioner.

In an alternate embodiment, three images are displayed on the samedisplay screen (or on separate screens but at the same time so that allare available to the system user at the same time). These three imagesare a single energy lateral scan image of a selected region of thepatient's spine, a dual energy lateral scan image of a selected regionof the patient's spine, and a single energy scan or a dual energy scantaken in a posterior/anterior or an anterior posterior view. A cursorcontroller such as a computer mouse, trackball or sole other deviceallows the system user to move a cursor on one of the displayed imaged.The three images and their display controllers are registered such thatany positioning or motion of the cursor on one of the images isautomatically and concurrently mirrored on the other displayed images.For example, if an operator of the system manually manipulates thecursor control to place the cursor so as to mark a reference point on anedge or vertebral body L4 on any one of the three images, e.g., thesingle energy lateral scan image, respective other cursors willautomatically mark the same point on the other two images of vertebralbody L4. As another example, if the operator places the cursor on the PAimage to the space separating L4 and L5, the system automatically placescursor at the corresponding point between L4 and L5 on the two lateralimages.

PA/Lateral Scan Measurement Processing

The separate scan lines of the PA and lateral scans can be matchedspatially to enhance the diagnostic value of the information theycontain. A PA scan typically is made and analyzed before performing asupine lateral scan. Once the PA scan is analyzed, the software executedby the densitometer computer system 38 can determine the center of thebone mass on each PA scan line, and then can determine an overallaverage center of the bone mass for the imaged portion of the spine.

In known densitometer systems, a relatively complicated computationalscheme has been utilized to match spatially the PA scan lines, involvinga best straight-line fit to the line-by-line bone mass centers. Whenperforming a subsequent supine lateral scan, the table and/or the C-armcan be moved so that this center of the bone mass for the respectivescan lines is positioned at a specific distance from the source.

The densitometer system of the present invention can eliminate the needto perform the best straight-line fit to the line-by-line bone masscenters. It incorporates absolute encoder positions for the patienttable and C-arm positions, and the arm starting encoder position isstored with the PA scan data. Before performing the lateral scan, thestarting encoder position is read from the data file, and the arm ismoved to a corresponding position for the lateral scan so that the APand lateral data are correctly aligned. This positional encodingtechnique allows the C-arm to be repositioned between the scans (e.g.,during movement from PA to lateral scan positions) without compromisingthe data alignment between the PA and lateral scans.

Multiple Pass Scans For Whole Body Measurements

The method is applicable to "whole body" scans acquired by fan beamx-ray analysis apparatus 30 of the present invention as previouslydescribed. Scans that are acquired in more than one longitudinal passalong the Y direction can have the data from the separate passescombined into an image corresponding to the image that would have beenobtained from a scan with a single wide x-ray beam. For a seamlessreconstruction, it is desirable that the passes with the narrower anglefan beam of x-rays 3a be aligned spatially and be free of geometricaldistortions. An accurate reconstruction can be accomplished if the x-raysystem provides the means to orient the source, detector and subjectsuch that there is an area of overlap between passes in which the x-raybeams are parallel and are attenuated in the same area of the subject.

A. Vertical Registration

In order to ensure accurate registration of the position of the C-arm 56and patient support table 50 with the acquired data, an electricalposition encoder is employed to provide position coordinates. In thelongitudinal (Y) direction, encoder positions are acquired along withcorresponding attenuation data during a scan. The encoder output isemployed by the computer 38 to align the individual scan measurementsafter acquisition. The averaged encoder positions are used to assign arelative shifts and the data reconstruction algorithm corrects thealignment by means of data shifting and interpolation.

B. Phase Alignment

Whole body scans using multiple passes are carried out in accordancewith the invention in a serpentine pattern. Time is not wasted inmotions that would otherwise be needed to start each pass at the same Yposition. For example, in a three pass whole body scan, the first passscans the right side of the patient from head to toe, the second passscans the patient's central region from toe to head, and the third passscans the patient's left side from head to toe. The x-ray systemgenerates multiple energy x-ray signals that are multiplexed in time, asseen in FIG. 14. An individual x-ray signal is referred to a phase, anda complete set of phases is referred to as a data line. In such asystem, at a given Y position the phase in one pass can be aligned witha different phase in the neighboring pass. In order to a match thephases so that data from adjacent passes can be correctly combined intoa single scan line, a new pass of data can be interpolated such that inthe X-direction, a scan line from one pass would match in phase the scanline from the adjacent pass. Thus, the scan lines of the individual scanpasses that are spatially aligned in the Y direction can be prepared tobe combined into a single scan data line.

C. Horizontal Registration

Although c-arm 64 and table 50 positions are calculated and controlledto produce passes that are aligned in the X direction with a knownamount of overlap, mechanical tolerances in the physical system canprevent perfect registration in the overlap region. In order to overcomethis physical limitation and minimize artifacts at the pass boundaries,the x-ray measurement data from the overlap region is used to determinethe actual amount of overlap in each scan. Adjustments are made byshifting the data points in the outer passes relative to the centralpass before the passes are combined.

The actual horizontal registration is determined by examining theoverlap region on each scan line. The overlap region is tested over arange of plus or minus half of the nominal overlap to find the shiftthat produces the maximum correlation and minimum accumulated absolutedifference between the sets of attenuation data in the two passes.Empirical thresholds are applied to the correlation coefficients anddifference sums to determine whether the overlap data for each scan lineis reliable. The result is a sparse array of shifts for each line.

The array of shifts for each line of the outer pass is smoothed andfilled or reduced to a zero or first order function to determine theexact shift for each line. If none of the data are reliable, then thenominal shift is used.

D. Scan Line Recombination

After the pass data has been registered properly, the opposing passlines are recombined into a single data line. Data in the overlap areasare blended to minimize artifacts at the interfaces between passes. Theblending is a point by point weighted average of the contributingpasses, with weights that are proportional to distance of each pointfrom the pass edge as a fraction of the overlap width.

E. Correction for Geometrical Effects

Recombined scan lines are equivalent to those that would have beenacquired with a single, wide fan beam projected to a three segmentdetector array, with the outer segments angled downward by the angle ofrotation. The effect is a compression of the image toward the scanedges. In order to correct the projection, each data line can bere-mapped, expanding the sides of the image. Correction of thedistortion can produce bone density and body composition measurementsthat are more accurate and uniform across the scan field. Aninterpolation map for this purpose can be calculated to normalize theprojected size of the x-ray detection face for each detector channel tothe size seen by an x-ray detector element in the center pass.

While a preferred embodiment of the invention has been described indetail, it should be understood that changes and variations will beapparent to those skilled in the art which are within the scope of theinvention recited in the appended claims.

Reconstructed Scan Image Processing and Manipulation

Scan data files include scan readings and accompanying positionalinformation obtained from the outputs of the patient support table 50and C-arm 56 position encoders (e.g., table XX translation systemencoder 202). Correlation of positional information with scaninformation is helpful for image processing and manipulation.

A reconstructed whole-body image, or selected portion thereof, aredisplayed on the monitor 44, and image processing software executed bythe host computer 38 is used to analyze bone mineral mass and density,bone surface area, and soft tissue composition including fat mass, leanmass and total mass.

User defined regions of interest (ROIs) can be placed in tile imagemanually, for example by use of a mouse of the host computer 38 (notshown), or automatically by the image processing software. ExemplaryROIs such as the spine, the proximal femur, the entire femur, the tibia,the head, the calcaneus, the hand, the fool and other bony structures,can be analyzed for bone mineral mass, bone surface area, and bonedensity. In addition, the patient's global bone mineral content, bonesurface area, and bone density can be obtained from the analysis of theentire whole-body scan image file.

Regional and global body composition analysis can also be performed onthe same image file by generating, either manually or automatically, thedesired ROIs within the image file. These ROIs yield information on thefat mass, lean mass and total mass of various body parts and regionsincluding the arms, legs, trunk, viscera, pelvis, thigh, chest, head andother regions.

Advantageously, a single image file can be created for a patient, whichcontains one or more clinically relevant anatomical regions. Regions ofinterest are generated either automatically or manually by the systemsoftware. Results of the various ROI analyses are stored with the imagefile, providing a convenient format whereby a single, automaticmeasurement of a selected region contains the raw x-ray and processeddata, including the measured bone mineral mass, bone mineral surfacearea, bone density and body composition data of one or more selected andclinically relevant anatomical sites.

Lateral projection whole-body scans can be performed for determiningdistribution of body fat in a patient. Such information may predictwhich subjects are at increased risk for various forms of cardiovasculardisease and cancer. The body fat distribution calculation is performedby taking a standard PA whole body measurement, utilizing the scannerdescribed in this specification. The source 52 and detector 54 are thenrotated ninety degrees with the c-arm 64 and a lateral whole-body imageis taken in a direction orthogonal to the AP scan. The AP and lateralimages are then processed to determine the distribution of fat mass,lean mass and total soft tissue mass within specific regions of thebody, including the pelvis, viscera, chest, upper thighs, arms and otherregions.

If scan sets are taken at multiple angles, tomographic images can bereconstructed in the manner known for CT scanning. The sets can beacquired by continuously rotating the source-detector support around thepatient, in the manner known for third generation CT scanners, or bymoving table 50 in the X direction while the source-detector support isstationary to thereby achieve motion of fan beam 2a equivalent to thatin second generation CT scanners, followed by a step rotation of thesource-detector support by the angle of beam 3a minus an overlap angle,followed by another motion of table 50 in the X direction, etc., stillin the manner known for second generation CT scanners.

CT Scanner Images From Bone Densitometry Scans

The system described above can generate tomographic images by collectingeither single or dual energy x-ray data while C-arm 56 rotatescontinuously or in steps while C-arm 56 and table 50 maintain a fixedrelative position along the Y-axis. In this manner, a single energy or adual energy CT image can be obtained and can be and analyzed for bonedensity on the same system that acquires AP and lateral bone densitydata, and dual energy can be used to acquire CT data with a fan beamthat is wide enough to encompass the spine but insufficiently wide toencompass the entire abdomen. This limited x-ray swath approach can beused to form an image of the bone only, and can be used for bone densitymeasurement of the spine. The resulting CT image reconstructs the bonestructure, but without showing soft tissue in the image, provided thatall of bone in the slice (the spine) is encompassed in the limited widthfan beam for all angles measured. Continuous or step-wise rotation ofC-arm 56 is accomplished while x-ray data is being collected. Rotationthrough an angle greater than 180° would generally be desirable forforming a CT image using well known image reconstruction mathematics.The system described here can allow a rotation through an angle of 100°,but alternatively can be arranged to allow rotation through 180° or evenmore. In the alternative, the system can acquire x-ray data while C-arm56 rotates through an angle of about 90°. The missing rays between 90°and 180° can be estimated by assuming they were equal to theircorresponding rays in the range of 0° to 90°. In this approach, thespine is assumed to have bilateral symmetry so that an x-ray measurementalong a ray at 10°. Whether the full complement of rays between 0° and180° are measured or the half-complement between 0° and 90° are measuredand the remaining half computed by symmetry, it is possible to acquire aseries of measurements to reconstruct a CT image as is known in the artusing a number of techniques such as filtered backprojection oralgebraic reconstruction. A desirable different technique is toreconstruct only the bone structure for lumbar vertebrae of the spine byusing the dual-energy x-ray measurements. Consider the x-raymeasurements as consisting of groups of parallel rays calledprojections. The set of measurement can then be described as a group ofprojections taken at different angles. The projections can be groupedaccording to their being composed of either rays of high energy x-raymeasurement or rays of low energy x-ray measurement. In a conventionalCT scanner, a given projection in general will contain rays that extendfrom one side of the patient to the other. But because the fan angle ofthe x-ray source in the preferred embodiment of the system describedhere does not encompass the entire abdomen of the patient, the rays willnot extend all the way to the sides of the patient. In order toreconstruct the bone structure with this limited fan angle system, thefollowing technique is used. For each projection the quantity Q=log H-klog L is formed, where k is a number equal to the ratio of theattenuation coefficient of non-bone tissue at the low and high energiesrespectively. The quantity Q is related to the bone density in theprojection (H is the logarithm of the high energy x-ray attenuation andL the logarithm of this low energy x-ray attenuation). When plotted inarbitrary unit of Q vs. distance across the patient along theX-direction, the general shape of the plot (in the X-direction) is arelatively flat line corresponding to tissue on one side of the spine(e.g., the left side) then a hump corresponding to the spine, thenanother relatively flat line for the tissue to the right of the spine,at approximately the same Q level as for the tissue on the left side.The soft tissue baseline may be set to zero for each projection. Theresulting "zeroed" baseline projections can then be used to form a CTimage of the bone structure alone using conventional CT reconstructionalgorithm. (The "zeroed" projections correspond to x-ray data that wouldbe needed to form a single energy CT image of the spine embedded in theair instead of tissue). In this technique, the disclosed system collectsdual energy x-ray projections over a limited view of the body whichincludes all of the bone in a slice but not all of the soft tissue,processes the dual energy x-ray measurements so that essentially softtissue is cancelled, and forms a CT image which reconstructs the bonestructure but not the soft tissue in a slice.

What is claimed is:
 1. A whole body x-ray bone densitometry systemcomprising:a table extending parallel to a Y-axis of an XYZ coordinatesystem for supporting a patient at a patient position; an x-ray sourcefor emitting a narrow angle fan beam of x-rays to irradiate at any onetime a scan line which is transverse to the Y-axis and is substantiallyshorter than the width of a body cross-section of a typical adultpatient occupying the patient position; an x-ray detector aligned withsaid source along a source-detector axis which is transverse to theY-axis, for receiving x-rays from the source within the angle of saidfan beam after passage thereof through the patient position, saiddetector comprising a number of detecting elements arranged along adirection transverse to the Y-axis and to the source-detector axis; asource-detector support on which the source and detector are mounted atopposite sides of the patient position; and a scanning mechanism movingthe patient table and the source-detector support relative to each otherparallel to the Y-axis to scan the patient position with said narrowangle fan beam in successive scans parallel to the Y-axis in which thesource-detector axis is at different angles relative to the patientposition as between different ones of said successive scans, but in eachof said successive scans an origin of the fan beam in the source is atthe same vertical distance from the patient table.
 2. A whole body x-raybone densitometry system comprising:a table extending parallel to aY-axis of an XYZ coordinate system for supporting a patient at a patientposition; an x-ray source for emitting a narrow angle fan beam of x-raysto irradiate at any one time a scan line which is transverse to theY-axis and is substantially shorter than the width of a bodycross-section of a typical adult patient occupying the patient position;an x-ray detector aligned with said source along a source-detector axiswhich is transverse to the Y-axis, for receiving x-rays from the sourcewithin the angle of said fan beam after passage thereof through thepatient position, said detector comprising a number of detectingelements arranged along a direction transverse to the Y-axis and to thesource-detector axis; a source-detector support on which the source anddetector are mounted at opposite sides of the patient position; and ascanning mechanism moving at least one of the patient table and thesource-detector support relative to the other to scan the patientposition with said narrow angle fan beam in successive scans parallel tothe Y-axis in which the source-detector axis is at different anglesrelative to the patient position as between different ones of saidsuccessive scans, but in each of said successive scans an origin of thefan beam in the source is at the same distance from a portion of thepatient table which is being irradiated by the x-ray beam.
 3. A wholebody x-ray bone densitometry system comprising:a table extendingparallel to a Y-axis of an XYZ coordinate system for supporting apatient at a patient position; an x-ray source for emitting a narrowangle fan beam of x-rays to irradiate at any one time a scan line whichis transverse to the Y-axis and is substantially shorter than the widthof a body cross-section of a typical adult patient occupying the patientposition; an x-ray detector aligned with said source along asource-detector axis which is transverse to the Y-axis, for receivingx-rays from the source within the angle of said fan beam after passagethereof through the patient position, said detector comprising a numberof detecting elements arranged along a direction transverse to theY-axis and to the source-detector axis; a source-detector support onwhich the source and detector are mounted at opposite sides of thepatient position; and a scanning mechanism moving the patient table andthe source-detector support relative to the other, including moving thepatient table parallel to the Y-axis, to scan the patient position withsaid narrow angle fan beam in successive scans parallel to the Y-axis inwhich the source-detector axis is at different angles relative to thepatient position as between different ones of said successive scans, butin each of said successive scans an origin of the fan beam in the sourceis at the same vertical distance from the patient table.
 4. A whole bodyx-ray bone densitometry system comprising:a table extending parallel toa Y-axis of an XYZ coordinate system for supporting a patient at apatient position; an x-ray source for emitting a narrow angle fan beamof x-rays to irradiate at any one time a scan line which is transverseto the Y-axis and is substantially shorter than the width of a bodycross-section of a typical adult patient occupying the patient position;an x-ray detector aligned with said source along a source-detector axiswhich is transverse to the Y-axis, for receiving x-rays from the sourcewithin the angle of said fan beam after passage thereof through thepatient position, said detector comprising a number of detectingelements arranged along a direction transverse to the Y-axis and to thesource-detector axis; a source-detector support on which the source anddetector are mounted at opposite sides of the patient position; and ascanning mechanism moving the patient table and the source-detectorsupport relative to the other, including moving the patient tableparallel to the Y-axis, to scan the patient position with said narrowangle fan beam in successive scans parallel to the Y-axis in which thesource-detector axis is at different angles relative to the patientposition as between different ones of said successive scans, but in eachof said successive scans an origin of the fan beam in the source is atthe same distance from a portion of the patient table which is beingirradiated by the x-ray beam.
 5. A system as in any one of claims 1-4,in which the scanning mechanism comprises a rotating mechanismselectively rotating the source-detector support within its own volumeof space, about a rotation axis parallel to the Y-axis betweensuccessive scans.
 6. A system as in any one of claims 1-4, wherein forsuccessive scans the scanning mechanism rocks the source-detectorsupport at different angles relative to the patient position about afixed point in space which is generally coincident with the origin ofthe fan beam in the source, and varies table height to maintain the samevertical distance between the origin of the fan in the source and thepatient table.
 7. A system as in claim 1, in which the scanningmechanism comprises a rotating mechanism selectively rotating thesource-detector support about a rotation axis parallel to the Y-axis andan elevating mechanism selectively varying patient table height parallelto the Z-axis and a translation mechanism selectively varying relativemotion between the source-detector support and the patient tableparallel to the X-axis.
 8. A system as in any one of claims 1-4, inwhich said scanning mechanism comprises a table moving mechanismselectively moving the table parallel to the X-axis between saidsuccessive scans.
 9. A system as in claim 8, in which said table movingmechanism comprises a mechanism selectively moving the table parallel tothe Z-axis between said successive scans.
 10. A system as in claim 9, inwhich said scanning mechanism comprises a rotating mechanism selectivelyrotating the source-detector support about a rotation axis parallel tothe Y-axis between said successive scans.
 11. A system as in claim 10,in which said scanning mechanism comprises a mechanism moving the tablebetween at least two of said successive scans through a motion having acomponent parallel to the X-axis over a distance DX and a componentparallel to the Z-axis over a distance DZ, where the distances DX and DZare according to the expressions:

    DX=(X.sub.2 -X.sub.1)=R cos φ(cosΘ-1)+sin φsin Θ!

    DZ=(Z.sub.2 -Z.sub.1)=R sin φ(cosΘ-1)-cos φsin Θ!

where: X₁ and X₂ are the positions of an origin of the fan shaped beamalong the X-axis before and after the table motion, Z₁ and Z₂ are thepositions of the origin along the Z-axis before and after the tablemotion, Θ is the negative of the angle through which the source-detectoraxis pivots between the at least two successive scans, φ is the anglethat the origin makes with the rotation axis at the end of the pivotingmotion of the source-detector support, and R is the distance between theorigin and the rotation axis.
 12. A system as in claim 11, in which saidangle Θ is about -21.5 degrees, with the negative angle denoting aclockwise rotation of the support about the rotation axis.
 13. A systemas in claim 12, in which the angle subsumed by said fan beam is about 22degrees, and the fan beam positions between two of said successive scansoverlap by about 0.5 degrees.
 14. A system comprising:a table forsupporting a patient at a patient position, said table having a lengthand a left side and a right side which are spaced from each other in adirection transverse to the length of the table, said table beingselectively movable at least along an axis aligned along its length,left-right and up-down; an x-ray source for emitting a fan beam ofx-rays; an x-ray detector receiving x-rays from the source within anangle subtended by said fan beam after passage thereof through thepatient position, said detector comprising a number of detectingelements aligned with respective angular positions in the fan beam andarranged along a direction transverse to the length of said table; asource-detector support supporting the source and detector at oppositesides of the patient position; and a scanning mechanism selectivelymoving the source-detector support along the length of the table androtating the source-detector support around the table within its ownvolume and selectively moving the table along the axis parallel to itslength and both left-right and up-down, to scan the patient positionwith said fan beam in successive scans along the length of the table inwhich the source-detector support is at different angles relative to thetable as between different ones of said successive scans but the up-downdistance between an origin of the fan beam in the source and the tableis the same for each of said successive scans.
 15. A system as in claim14, in which the scanning mechanism comprises a table moving mechanismwhich between said successive scans moves the table left-right andup-down and rotates the source-detector support about a rotation axisthat does not pass through the origin of the fan beam.
 16. A system asin claim 14, in which said fan beam at said patient position is narrowerthan the width of a typical adult patient who is in a supine position onthe table.
 17. A system as in claim 16, in which said beam irradiatesoverlapping portions of said patient position in said successive scans.18. A system as in claim 14, in which said scanning mechanism comprisesa mechanism carrying out said successive scans for an anterior-posteriorview of the patient position, and selectively changing the up-downdistance between said origin and the table and scanning the beamrelative to the patient position for a lateral view without moving thepatient relative to the table.
 19. A system as in claim 18, in whichsaid scanning mechanism additionally comprises a mechanism for changingthe up-down distance between said origin and the table and scanning thepatient with the fan beam for a hip view without moving the patientrelative to the table.
 20. A system as in claim 14, in which saidscanning mechanism comprises a mechanism carrying out said successivescans for an anterior-posterior view of a patient and selectivelychanging the up-down distance between said origin and the table andscanning the beam relative to the patient position for a lateral viewand a hip view without moving the patient relative to the table.
 21. Amethod comprising:placing a patient on a table; irradiating the patientwith a fan beam of x-rays subtending an angle that includessubstantially less than the body width of the patient; receiving x-raysfrom the source within the angle subtended by the fan beam after passagethereof through the patient at a number of radiation detecting positionsaligned with angular positions within said angle; scanning the fan beamand the detector along the length of the patient in successive scans bymoving the table and the fan beam relative to each other parallel to theY-axis and selectively moving at least one of the table and the fan beamalong each of two orthogonal axes transverse to the length of thepatient between said successive scans to scan the patient from differentangles while maintaining a selected vertical distance between an originof the fan beam and the table.
 22. A method comprising:placing a patienton a table irradiating the patient with a fan beam of x-rays subtendingan angle that includes substantially less than the body width of thepatient; receiving x-rays from the source within the angle subtended bythe fan beam after passage thereof through the patient at a number ofradiation detecting positions aligned with angular positions within saidangle; scanning the fan beam and the detector along the length of thepatient in successive scans and selectively moving at least one of thetable and the fan beam along each of two orthogonal axes transverse tothe length of the patient between said successive scans to scan thepatient from different angles while maintaining a selected verticaldistance between an origin of the fan beam and a portion of the patienttable which is being irradiated by the beam.
 23. A methodcomprising:placing a patient on a table irradiating the patient with afan beam of x-rays subtending an angle that includes substantially lessthan the body width of the patient; receiving x-rays from the sourcewithin the angle subtended by the fan beam after passage thereof throughthe patient at a number of radiation detecting positions aligned withangular positions within said angle; scanning the fan beam and thedetector along the length of the patient, including moving the tablerelative to the patient's length, in successive scans and selectivelymoving at least one of the table and the fan beam along each of twoorthogonal axes transverse to the length of the patient between saidsuccessive scans to scan the patient from different angles whilemaintaining a selected vertical distance between an origin of the fanbeam and the table.
 24. A method comprising:placing a patient on atable; irradiating the patient with a fan beam of x-rays subtending anangle that includes substantially less than the body width of thepatient; receiving x-rays from the source within the angle subtended bythe fan beam after passage thereof through the patient at a number ofradiation detecting positions aligned with angular positions within saidangle; scanning the fan beam and the detector along the length of thepatient, including moving the table relative to the patient's length, insuccessive scans and selectively moving at least one of the table andthe fan beam along each of two orthogonal axes transverse to the lengthof the patient between said successive scans to scan the patient fromdifferent angles while maintaining a selected vertical distance betweenan origin of the fan beam and a portion of the patient table which isbeing irradiated by the beam.
 25. A method as in any one of claims21-24, in which the fan beam in said successive scans irradiatesoverlapping portions of the patient to generate detector measurementsfor an anterior-posterior view of the patient, and additionallycomprising the step of changing the vertical distance between the originand the table and scanning the beam relative to the patient to obtain alateral view without moving the patient relative to the table.
 26. Amethod as in claim 25, further comprising changing the vertical distancebetween the origin and the table and scanning the beam relative to thepatient to obtain an oblique hip view without moving the patientrelative to the table.
 27. A method as in claim any one of claims 21-24,further comprising changing the vertical distance between the origin andthe table and scanning the beam relative to the patient to obtain anoblique hip view without moving the patient relative to the table.
 28. Amethod comprising:supporting a patient with the length thereof beingalong the Y-axis of an X,Y,Z orthogonal set of axes; emitting a fan beamof x-rays from an origin to irradiate at any one time less than theentire width of the patient; receiving x-rays from the source within anangle subtended by the fan beam after passage thereof through thepatient at a number of individual detecting positions arranged along aplane normal to the Y-axis; moving at least one of the patient table andthe combination of fan beam and the detector concomitantly to scan thepatient in successive scans which are along the Y-axis but in which thefan beam is at different angles to the patient, rotating the fan beamabout a rotation axis parallel to the Y-axis between said successivescans, and moving the table along the X-axis and the Z-axis to maintaina selected vertical distance between the origin and the table to obtaindetector outputs for an anterior-posterior view of the entire width ofthe patient while maintaining the patient's position relative to thetable.
 29. A method as in claim 28, further comprising selectivelychanging the vertical distance between the origin and the table and theangle of the beam to the patient and scanning the beam relative to thepatient to obtain detector outputs for a lateral view without moving thepatient relative to the table.
 30. A method as in claim 28, furthercomprising selectively changing the vertical distance between the originand the table and the angle of the beam to the patient and scanning thebeam relative to the patient to obtain detector outputs for a hip viewwithout moving the patient relative to the table.
 31. A method as inclaim 28, further comprising the steps of selectively changing thevertical distance between the origin and the table and the angle of thebeam to the patient and scanning the beam relative to the patient toobtain detector outputs for a lateral view and a hip view without movingthe patient relative to the table.
 32. A method as in claim 28, furthercomprising processing detector outputs to form and display x-ray imagesof the patient.
 33. A whole body x-ray bone densitometry systemcomprising:a table extending in a direction parallel to a Y-axis of anXYZ coordinate system for supporting a patient at a patient position,and movable in a direction which is parallel to at least one of the X-,Y- and Z-axes; an x-ray source emitting a narrow angle fan beam ofx-rays from a focal point of origin to irradiate at any one time a scanline which is transverse to the Y-axis and is substantially shorter thanthe width of a body cross-section of a typical adult patient occupyingthe patient position; an x-ray detector aligned with said source along asource-detector axis which is transverse to the Y-axis, for receivingx-rays from the source within the angle of said fan beam after passagethereof through the patient position, said detector comprising a numberof detecting elements arranged along a direction transverse to theY-axis and to the source-detector axis; a source-detector support onwhich the source and detector are mounted at opposite sides of thepatient position; and a scanning mechanism moving at least one of thepatient table and the source-detector support relative to the other toscan the patient position with said narrow angle fan beam in successivescans parallel to the Y-axis in which the source-detector axis is atdifferent angles relative to the patient position as between differentones of said successive scans; said scanning mechanism causing thedistance along the Z-axis between the focal point of origin and thepatient table to differ at least between two of said successive passes.34. A whole body x-ray bone densitometry system comprising:a tableextending in a direction parallel to a Y-axis of an XYZ coordinatesystem for supporting a patient at a patient position, and movable in adirection which is parallel to at least one of the X-, Y- and Z-axes; anx-ray source emitting a narrow angle fan beam of x-rays from a focalpoint of origin to irradiate at any one time a scan line which istransverse to the Y-axis and is substantially shorter than the width ofa body cross-section of a typical adult patient occupying the patientposition; an x-ray detector aligned with said source along asource-detector axis which is transverse to the Y-axis, for receivingx-rays from the source within the angle of said fan beam after passagethereof through the patient position, said detector comprising a numberof detecting elements arranged along a direction transverse to theY-axis and to the source-detector axis; a source-detector support onwhich the source and detector are mounted at opposite sides of thepatient position; and a scanning mechanism moving at least one of thepatient table and the source-detector support relative to the other toscan the patient position with said narrow angle fan beam in successivescans parallel to the Y-axis in which the source-detector axis is atdifferent angles relative to the patient position as between differentones of said successive scans; said scanning mechanism causing thedistance between the focal point of origin and a portion of the patienttable which is being irradiated by the x-ray beam to differ at leastbetween two of said successive passes.
 35. A whole body x-ray bonedensitometry system comprising:a table extending in a direction parallelto a Y-axis of an XYZ coordinate system for supporting a patient at apatient position; an x-ray source emitting a narrow angle fan beam ofx-rays from a focal point of origin to irradiate at any one time a scanline which is transverse to the Y-axis and is substantially shorter thanthe width of a body cross-section of a typical adult patient occupyingthe patient position; an x-ray detector aligned with said source along asource-detector axis which is transverse to the Y-axis, for receivingx-rays from the source within the angle of said fan beam after passagethereof through the patient position, said detector comprising a numberof detecting elements arranged along a direction transverse to theY-axis and to the source-detector axis; a source-detector support onwhich the source and detector are mounted at opposite sides of thepatient position; and a scanning mechanism moving at least one of thepatient table and the source-detector support relative to the other toscan the patient position with said narrow angle fan beam in successivescans parallel to the Y-axis in which the source-detector axis is atdifferent angles relative to the patient position as between differentones of said successive scans; said scanning mechanism causing thedistance along the Z-axis between the focal point of origin and thepatient table to differ selectively at least between two of saidsuccessive passes.
 36. A whole body x-ray bone densitometry systemcomprising:a table extending in a direction parallel to a Y-axis of anXYZ coordinate system for supporting a patient at a patient position; anx-ray source emitting a narrow angle fan beam of x-rays from a focalpoint of origin to irradiate at any one time a scan line which istransverse to the Y-axis and is substantially shorter than the width ofa body cross-section of a typical adult patient occupying the patientposition; an x-ray detector aligned with said source along asource-detector axis which is transverse to the Y-axis, for receivingx-rays from the source within the angle of said fan beam after passagethereof through the patient position, said detector comprising a numberof detecting elements arranged along a direction transverse to theY-axis and to the source-detector axis; a source-detector support onwhich the source and detector are mounted at opposite sides of thepatient position; and a scanning mechanism moving at least one of thepatient table and the source-detector support relative to the other toscan the patient position with said narrow angle fan beam in successivescans parallel to the Y-axis in which the source-detector axis is atdifferent angles relative to the patient position as between differentones of said successive scans; said scanning mechanism causing thedistance between the focal point of origin and a portion of the patienttable which is being irradiated by the x-ray beam to differ selectivelyat least between two of said successive passes.
 37. A whole body x-raybone densitometry system as in any one of claims 33-36, wherein saidsuccessive scans include a central scan and additional scans, and saiddistance is less for the central scan than for the additional scans. 38.A whole body x-ray bone densitometry system as in any one of claims33-36, wherein said successive scans include a central scan andadditional scans, and said distance is greater for the central scan thanfor the additional scans.
 39. A whole body x-ray bone densitometrysystem as in any one of claims 33-36, wherein said beam irradiatesoverlapping portions of said patient position in said successive scans.40. The whole body x-ray bone densitometry system of claim 39, whereinthe overlapping portions are parallel.
 41. A whole body x-ray bonedensitometry system as in any one of claims 33-36, wherein said beamirradiates abutting, but not overlapping, portions of said patientposition in said successive scans.
 42. A whole body x-ray bonedensitometry method comprising:supporting a patient supine on a movabletable, at a patient position; emitting a fan beam of x-rays from anorigin in a source at one side of the patient and receiving x-rays fromthe source at an x-ray detector which is at an opposite side of thepatient; supporting the source and detector on a source-detector supportfor movement as a unit relative to the patient; and moving at least oneof the patient table and the source-detector support relative to theother to scan the patient in successive scans which are along matchingdirections but in which the fan beam is at an angle relative to the fanbeam for other scans; wherein the vertical distance between the originof the fan beam and the patient table differs at least as between two ofsaid successive scans.
 43. A whole body x-ray bone densitometry methodcomprising:supporting a patient supine on a table, at a patientposition; emitting a fan beam of x-rays from an origin in a source atone side of the patient and receiving x-rays from the source at an x-raydetector which is at an opposite side of the patient; supporting thesource and detector on a source-detector support for movement as a unitrelative to the patient; and moving at least one of the patient tableand the source-detector support relative to the other to scan thepatient in successive scans which are along matching directions but inwhich the fan beam is at an angle relative to the fan beam for otherscans; wherein the vertical distance between the origin of the fan beamand the patient table selectively differs at least as between two ofsaid successive scans.
 44. A whole body x-ray bone densitometry methodcomprising:supporting a patient supine on a movable table, at a patientposition; emitting a fan beam of x-rays from an origin in a source atone side of the patient and receiving x-rays from the source at an x-raydetector which is at an opposite side of the patient; supporting thesource and detector on a source-detector support for movement as a unitrelative to the patient; and moving at least one of the patient tableand the source-detector support relative to the other to scan thepatient in successive scans which are along matching directions but inwhich the fan beam is at an angle relative to the fan beam for otherscans; wherein the distance between the origin of the fan beam and aportion of the patient table which is being irradiated by the fan beamdiffers at least as between two of said successive scans.
 45. A wholebody x-ray bone densitometry method comprising:supporting a patientsupine on a table, at a patient position; emitting a fan beam of x-raysfrom an origin in a source at one side of the patient and receivingx-rays from the source at an x-ray detector which is at an opposite sideof the patient; supporting the source and detector on a source-detectorsupport for movement as a unit relative to the patient; and moving atleast one of the patient table and the source-detector support relativeto the other to scan the patient in successive scans which are alongmatching directions but in which the fan beam is at an angle relative tothe fan beam for other scans; wherein the distance between the origin ofthe fan beam and a portion of the patient table which is beingirradiated by the fan beam selectively differs at least as between twoof said successive scans.
 46. A whole body x-ray bone densitometrymethod as in any one of claims 42 -45, wherein said successive scansinclude a central scan and additional scans, and said distance is lessfor the central scan than for the additional scans.
 47. A whole bodyx-ray bone densitometry method as in any one of claims 42-54, whereinsaid successive scans include a central scan and additional scans, andsaid distance is greater for the central scan than for the additionalscans.
 48. A whole body x-ray bone densitometry method as in any one ofclaims 42-45, wherein said beam irradiates overlapping portions of saidpatient position in said successive scans.
 49. A whole body x-ray bonedensitometry method as in any one of claims 42-45, wherein theoverlapping portions are parallel.
 50. A whole body x-ray bonedensitometry method as in any one of claims 42-45, wherein said beamirradiates abutting, but not overlapping, portions of said patientposition in said successive scans.
 51. An x-ray bone densitometryapparatus as in any one of claims 1-4, 14, and 33-36, furthercomprising:a scan motion controller moving at least one of thesource-detector support and the patient table relative to the other toscan at least a first region and then a second region of a patient onthe table with the beam parallel to the Y-axis; said first region andsaid second region being next to each other along the X-axis, eachhaving an edge overlapping an adjacent edge of the other region; thedistance along the Z-axis from an origin of the fan beam in the sourceto the table being the same for the scan of each of the first and secondregions.
 52. An x-ray bone densitometry apparatus as in claim 51 furthercomprising:a merger responsive to outputs of the detector for scans ofthe first and second regions to merge detector outputs for positions ofthe fan beam which are spatially adjacent along the X-axis but areobtained at different times, into resulting merged detector outputscorresponding to detector outputs obtainable from a single fan beamhaving substantially twice the width of the fan beam emitted from thesource.
 53. An x-ray bone densitometry apparatus as in any one of claims1-4, 14, and 33-36, further comprising:a scan motion controller movingat least one of the source-detector support and the table relative tothe other to carry out a first posterior/anterior or ananterior/posterior scan and then a lateral scan of a patient on thetable without moving the patient between the two views; said scan motioncontroller rotating the source-detector support around a rotation axisthereof between the two scans; and said scan motion controller selectingthe position of the source relative to the table for the lateral scanbased on information respecting the patient's spine obtained in thecourse of the anterior/posterior or posterior/anterior scan.